What is the study of the distribution and dynamics of diseases in populations?

Descriptive Epidemiology

Introduction

The image above illustrates the ten essential functions of public health. Epidemiology plays a particularly important role for three of the functions: monitoring, investigating, and evaluating. The 10 Essential Public Health Services describe the public health activities that all communities should undertake. Public health systems should

1. Monitor health status to identify and solve community health problems.
2. Diagnose and investigate health problems and health hazards in the community.
3. Inform, educate, and empower people about health issues.
4. Mobilize community partnerships and action to identify and solve health problems.
5. Develop policies and plans that support individual and community health efforts.
6. Enforce laws and regulations that protect health and ensure safety.
7. Link people to needed personal health services and assure the provision of health care when otherwise unavailable.
8. Assure competent public and personal health care workforce.
9. Evaluate effectiveness, accessibility, and quality of personal and population-based health services.
10. Research for new insights and innovative solutions to health problems.

Disease surveillance systems and health data sources provide the raw information necessary to monitor trends in health and disease. Descriptive epidemiology provides a way of organizing and analyzing these data in order to understand variations in disease frequency geographically and over time, and how disease (or health) varies among people based on a host of personal characteristics (person, place, and time). This makes it possible to identify trends in health and disease and also provides a means of planning resources for populations. In addition, descriptive epidemiology is important for generating hypotheses (possible explanations) about the determinants of health and disease. By generating hypotheses, descriptive epidemiology also provides the starting point for analytic epidemiology, which formally tests associations between potential determinants and health or disease outcomes. Specific tasks of descriptive epidemiology are the following:

• Monitoring and reporting on the health status and health related behaviors in populations
• Identifying emerging health problems
• Alerting us to potential threats from bioterrorism
• Establishing public health priorities for a population
• Evaluating the effectiveness of intervention programs and
• Exploring potential associations between "risk factors" and health outcomes in order to generate hypotheses about the determinants of disease.

Learning Objectives

After successfully completing this unit, the student will be able to:

• Explain the role of descriptive studies for identifying problems and establishing hypotheses.
• Explain how the characteristics of person, place, & time are used to formulate hypotheses in acute disease outbreaks and in studies of chronic diseases.
• Identify case reports and case series and explain their uses and their limitations.
• Describe the design features of an ecologic study and discuss their strengths and weaknesses.
• Explain the concept of ecologic fallacy both in general and in the context of a study. Identify the strengths and limitations of an ecologic study.
• Describe the design features of a cross-sectional study and describe their uses, strengths, and limitations.

Hypothesis Formulation – Characteristics of Person, Place, and Time

Descriptive epidemiology searches for patterns by examining characteristics of person, place, & time. These characteristics are carefully considered when a disease outbreak occurs, because they provide important clues regarding the source of the outbreak.

Hypotheses about the determinants of disease arise from considering the characteristics of person, place, and time and looking for differences, similarities, and correlations. Consider the following examples:

• Differences: if the frequency of disease differs in two circumstances, it may be caused by a factor that differs between the two circumstances. For example, there was a substantial difference in the incidence of stomach cancer in Japan & the US. There are also substantial differences in genetics and diet. Perhaps these factors are related to stomach cancer.

• Similarities: if a high frequency of disease is found in several different circumstances & one can identify a common factor, then the common factor may be responsible. Example: AIDS in IV drug users, recipients of transfusions, & hemophiliacs suggests the possibility that HIV can be transmitted via blood or blood products.

• Correlations: If the frequency of disease varies in relation to some factor, then that factor may be a cause of the disease. Example: differences in coronary heart disease vary with cigarettes consumption.

Descriptive epidemiology provides a way of organizing and analyzing data on health and disease in order to understand variations in disease frequency geographically and over time and how disease varies among people based on a host of personal characteristics (person, place, and time). Epidemiology had its origins in the desire to understand the determinants of acute infectious diseases, but its methods and applicability have expanded to include chronic diseases as well.

Descriptive Epidemiology for Infectious Disease Outbreaks

Outbreaks generally come to the attention of state or local health departments in one of two ways:

1. Astute individuals (citizens, physicians, nurses, laboratory workers) will sometimes notice cases of disease occurring close together with respect to time and/or location or they will notice several individuals with unusual features of disease and report them to health authorities.
2. Public health surveillance systems collect data on 'reportable diseases'. Requirements for reporting infectious diseases in Massachusetts are described in 105 CMR 300.000 (Link to Reportable Diseases, Surveillance, and Isolation and Quarantine Requirements).

Clues About the Source of an Outbreak of Infectious Disease

When an outbreak occurs, one of the first things that should be considered is what is known about that particular disease. How can the disease be transmitted? In what settings is it commonly found? What is the incubation period? There are many good summaries available online. For example, Massachusetts DPH provides this link to a PDF fact sheet for Hepatitis A, which provide a very succinct summary. With this background information in mind, the initial task is to begin to characterize the cases in terms of personal characteristics, location, and time (when did they become ill and where might they have been exposed given the incubation period for that disease. In sense, we are looking for the common element that explains why all of these people became ill. What do they have in common?

"Person"

Information about the cases is typically recorded in a "line listing," a grid on which information for each case is summarized with a separate column for each variable. Demographic information is always relevant, e.g., age, sex, and address, because they are often the characteristics most strongly related to exposure and to the risk of disease. In the beginning of an investigation a small number of cases will be interviewed to look for some common link. These are referred to as "hypothesis-generating interviews." Depending on the means by which the disease is generally transmitted, the investigator might also want to know about other personal characteristics, such as travel, occupation, leisure activities, use of medications, tobacco, drugs. What did these victims have in common? Where did they do their grocery shopping? What restaurants had they gone to in the past month or so? Had they traveled? Had they been exposed to other people who had been ill? Other characteristics will be more specific to the disease under investigation and the setting of the outbreak. For example, if you were investigating an outbreak of hepatitis B, you should consider the usual high-risk exposures for that infection, such as intravenous drug use, sexual contacts, and health care employment. Of course, with an outbreak of foodborne illness (such as hepatitis A), it would be important to ask many questions about possible food exposures. Where do you generally eat your meals? Do you ever eat at restaurants or obtain foods from sources outside the home? Hypothesis generating interviews may quickly reveal some commonalities that provide clues about the possible sources.

"Place"

Assessment of an outbreak by place provides information on the geographic extent of a problem and may also show clusters or patterns that provide clues to the identity and origins of the problem. A simple and useful technique for looking at geographic patterns is to plot, on a "spot map" of the area, where the affected people live, work, or may have been exposed. A spot map of cases may show clusters or patterns that reflect water supplies, wind currents, or proximity to a restaurant or grocery store.

In 1854 there was an epidemic of cholera in the Broad Street area of London. John Snow determined the residence or place of business of the victims and plotted them on a street map (the stacked black disks on the map below). He noted that the cases were clustered around the Broad Street community pump. It was also noteworthy that there were large numbers of workers in a local workhouse and a brewery, but none of these workers were affected - the workhouse and brewery each had their own well.

On a spot map within a hospital, nursing home, or other such facility, clustering usually indicates either a focal source or person-to-person spread, while the scattering of cases throughout a facility is more consistent with a common source such as a dining hall. In studying an outbreak of surgical wound infections in a hospital, we might plot cases by operating room, recovery room, and ward room to look for clustering.

• Link to more on the outbreak of cholera in the Broad Street area of London
• Link to an enlarged version of Snow's spot map

"Time"

When investigating the source of an outbreak of infectious disease, Investigators record the date of onset of disease for each of the victims and then plot the onset of new cases over time to create what is referred to as an epidemic curve. The epidemic curve for an outbreak of hepatitis A is shown in the illustration below. Begriming in late April, the number of new cases rises to a peak of twelve new cases reported on May 12, and then the number of new cases gradually drops back to zero by May 21. Knowing that the incubation period for hepatitis A averages about 28-30 days, the investigators concluded that this was a point source epidemic because the cluster of new cases all occurred within the span of a single incubation period (see explanation on the next page). This, in conjunction with other information, provided important clues that helped shape their hypotheses about the source of the outbreak.

Video Summary: Person, Place, and Time (10:42)

Epidemic Curves

An "epidemic curve" shows the frequency of new cases over time based on the date of onset of disease. The shape of the curve in relation to the incubation period for a particular disease can give clues about the source. There are three basic types of epidemic curve.

Point source outbreaks (epidemics) involve a common source, such as contaminated food or an infected food handler, and all the exposures tend to occur in a relatively brief period. Consequently, point source outbreaks tend to have epidemic curves with a rapid increase in cases followed by a somewhat slower decline, and all of the cases tend to fall within one incubation period.  The graph above from a hepatitis outbreak is an example of a point source epidemic. The incubation period for hepatitis ranges from 15-50 days, with an average of about 28-30 days. In a point source epidemic of hepatitis A you would expect the rise and fall of new cases to occur within about a 30 day span of time, which is what is seen in the graph below.

Continuous common source epidemics may also rise to a peak and then fall, but the cases do not all occur within the span of a single incubation period. This implies that there is an ongoing source of contamination. The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself. The epidemic curve below is from the cholera outbreak in the Broad Street area of London in 1854 that was investigated by Dr. John Snow. Cholera has an incubation period of 1-3 days, and even though residents began to flee when the outbreak erupted, you can see that this outbreak lasted for more than a single incubation period. This suggests an ongoing source of infection, in this case the Broad Street pump.

Propagated (or progressive source) epidemic. The epidemic curve shown below is from an outbreak of measles that began with a single index case who infected a number of other individuals. (The incubation period for measles averages 10 days with a range of 7-18 days.) One or more of the people infected in the initial wave infected a group of people who become the second wave of infection. So here transmission is person-to-person, rather than from a common source. Propagated epidemic curves usually have a series of successively larger peaks, which are one incubation period apart. The successive waves tend to involve more and more people, until the pool of susceptible people is exhausted or control measures are implemented. This is an ideal example, however; in reality, most of these epidemics do not produce the classic pattern.

For some outbreaks the descriptive information is all that is needed to figure out the source, and control measures can be undertaken rapidly. In other cases, this descriptive information (person, place, and time) helps generate hypotheses about the source, but it isn't obvious what the source is. When this occurs, it is necessary to test the hypotheses by conducting an analytical study, i.e. either a case-control study or a cohort study. This means collecting data and analyzing it in order to identify the source. After the hepatitis outbreak in Marshfield, DPH conducted a case-control study. After an outbreak of Giardia in Milton, MA, a retrospective cohort study was conducted. However, it is important to recognize that you can't test a hypothesis unless you have one to test. So, the descriptive studies that generate hypotheses are essential.

Use the graph below to answer this "Quiz Me."

(Optional) - Two Methods for Creating an Epidemic Curve in Excel

Method 1 - video

Method 2 - video

(Optional) - Steps in the Investigation of a Disease Outbreak

Most outbreak investigations involve the following steps:

1. Preparation for the investigation
2. Verifying the diagnosis and establishing the existence of an outbreak
3. Establishing a case definition and finding cases
4. Conducting descriptive epidemiology to determine the personal characteristics of the cases, changes in disease frequency over time, and differences in disease frequency based on location.
5. Developing hypotheses about the cause or source
6. Evaluating the hypotheses & refining the hypotheses and conducting additional studies if necessary
7. Implementing control and prevention measures
8. Communicating the findings

Some of these steps may be conducted simultaneously, and the order may vary depending on the circumstances. For example, if new cases are continuing to occur and there are steps that can be taken to control the outbreak and prevent more cases, then certainly control and prevention measures would take top priority.

Optional Additional Resources

General Information on Outbreak Investigations

For an overview of outbreak investigations for foodborne illness see the CDC web page linked here. Other good general sources of information on how to conduct outbreak investigations can be found in the University of North Carolina (UNC) online Focus on Field Epidemiology series. The following links to online articles may be of interest:

Volume 1

• Issue #1: Overview of Outbreak Investigations

• Issue #2: Anatomy and Physiology of an Outbreak Team

• Issue #3: Embarking on an Outbreak Investigation

• Issue #4: Case Finding and Line Listing: A Guide for Investigators

• Issue #5: Epidemic Curves Ahead with a Focus Flash on Creating an Epidemic Curve in Excel

• Issue #6:Hypothesis Generation During Outbreaks

Volume 2:

• Issue #1: Hypothesis-Generating Interviews

• Issue #2: Developing a Questionnaire

• Issue #3: Interviewing Techniques

Another good general resource is "Hepatitis in Sparta." This is an online interactive teaching case that thrusts the student into the role of investigator trying to determine the source for an outbreak of hepatitis cases in the town of Sparta.

Descriptive Epidemiology for Chronic Diseases

The same questions about person, time, and place can be applied to chronic diseases.  Who are the people who have the disease? What are their characteristics? What is their occupation? Where do they live and work? How did disease occurrence vary over time?

Personal Characteristics

Personal characteristics also provide clues about the causes of chronic diseases. Many disease vary in relation to age and gender, but many other characteristics are also important, such as occupation, diet, sexual activity, travel history, and personal behaviors (exercise, smoking, etc.)

Age-specific Rates of Disease

Because so many diseases vary in relation to disease, one frequently sees disease rates categorized this way - so-called "age-specific rates of disease." Mortality rates are very low in the youngest age groups & similar in males and females. In adulthood the mortality rates rise sharply and become higher in males. Although the mortality rate continues to rise into old age, the gender difference begins to narrow. One might describe this as a chronic, progressive disease in which the gender differences raise the question of whether sex hormones play a role, particularly since females begin to catch up after menopause occurs.

Table - Death Rates from Coronary Artery Disease (Age-Specific Rates)

Differences by Race and Ethnicity

In addition to age and gender one might want to examine how disease rates differ with respect to other characteristics, such as race. The table below summarizes. annual mortality rates per 100,000 in whites and blacks in the United States in 1967. Ethnic and racial differences in disease rates sometimes have a genetic basis, e.g., sickle cell anemia in people of African descent or beta thalassemia in people of Mediterranean descent, but in other cases racial differences are due to environmental or socioeconomic factors.

• Link to more on sickle cell anemia
• Link to more on beta thalassemia

Table - Annual Mortality Rates per 100,000 population in the US, 1967

Other Personal Characteristics

Besides age, gender and race/ethnicity, other personal characteristics that might be important to consider are:

• Occupation
• Diet
• Religious practices, e.g. dietary restrictions or restrictions on drinking alcohol or tobacco use
• Leisure activities, e.g., exercise

Place: Variation by Location

Differences in disease frequency by location provides important clues about the determinants of chronic diseases. Where does the disease tend to occur?

• Does the frequency of disease vary from country to country? Or state to state?
• Does it vary among cities or neighborhoods?
• Does it vary within different parts of a large workplace?

Example 1: Stomach Cancer by Location in the US

These maps show death rates from stomach cancer in females (top) and males (below) in different US counties. The darkness of shading of each county indicates how its stomach cancer rate compares with the national average. The darkest shading indicates rates well above average, and white shading indicates rates below average; the gray shading indicates intermediate levels. Note that rates of stomach cancer tend to be high in counties in the north-central part of the country in both males and females. Investigators speculated that these clusters might correlate with populations of German or Scandinavian descent who have a tradition of eating smoked fish. Could the high rates of stomach cancer be the result of their consumption of smoked fish or other traditional methods of food preservation?

Source: Atlas of Cancer Mortality for U.S. Counties: 1950-1969, TJ Mason et al, PHS, NIH, 1975

Example 2: Differences in Rates of Stomach Cancer in Japan and US

Rates of stomach cancer also vary among countries. Japanese have a higher rate of stomach cancer than Caucasians in California. Is this due to a genetic difference? A dietary difference? The rate among Japanese people diminishes after they move to US, and diminishes even more in their offspring. One possibility is that once the Japanese move here, they begin to shift to an American diet, and this trend is even stronger in their children. Are there important dietary differences? Could consumption of large amounts of smoked fish be a cause of stomach cancer?

Variation in Disease Over Time

• Has the frequency of disease changed over several decades?
• Does frequency of disease vary in a cyclic way that relates to the seasons?
• Has it changed over the course of days?

Changes in disease rate over time can also provide clues for chronic diseases.

Example 1: Annual Mortality from Pulmonary Tuberculosis in England and Wales

Tuberculosis (TB) is one of the great killers of all times. The graph on the right shows the mortality rate from TB from 1855-1955 in England and Wales. The remarkable downward trend began well before the development of antibiotics. The steady improvement was probably a direct result of "the sanitary idea" which resulted in concerted efforts to improve working and living conditions, nutrition, ventilation, and waste management. Also, note the increases in TB mortality that occurred during World War I and World War II. This suggests that nutritional deficiencies, translocation, crowding, and other adverse circumstances associated with war are contributing factors to the causation of TB.

Example 2: Toxic Shock and Rely Tampons

In January 1980 there were several reports of toxic shock syndrome due to infection with Staphylococcus aureus bacteria, and the descriptive epidemiology indicated that the problem was occurring primarily in menstruating women. A CDC task force investigated and eventually traced the outbreak to the introduction of Rely tampons, a super absorbent product marketed by Proctor and Gamble. The monthly cases of toxic shock syndrome in 1980-1981 are shown in the graph below [from A. Reingold et al., Toxic shock syndrome surveillance in the United States, 1980-1981. Ann. Intern. Med 96:875, 1982]. The graph shows that prior to 1978 there were just occasional cases of toxic shock syndrome in the United States. After Rely tampons were introduced in 1978, there was a steady increase in toxic shock cases which peaked at about 125 per month in 1980. Shortly after that, Rely tampons were taken off the market, and the incidence declined sharply.

There were actual two pieces of evidence related to time variations that supported Rely tampons as the cause. First, descriptive epidemiology suggested a link to menstruation, leading doctors to take bacterial cultures from the vagina. This provided a key clue suggesting a link to certain brands of tampons. In addition, the frequency of toxic shock syndrome clearly correlated with the introduction and subsequent removal of Rely tampons from the market.

• Link to more on toxic shock syndrome

Other Factors That Can Produce Changes in Disease Frequency Over Years or Decades

If the frequency of a disease or mortality from a disease changes over time, there are several factors which could be responsible:

• Changes in incidence due to environmental or life-style changes.
• Improvements in diagnosis may increase cases reported even though the incidence may not be changing.
• Changes in record keeping (accuracy) can create what appear to be changes in disease rates.
• Improved treatment may decrease mortality rates
• Changes in the age distribution of a population can produce changes in the overall rate of disease, even though age-specific rates are not changing.

Two Fundamental Types of Study Questions

Specifying the research questions is essential to selection of an appropriate study population, and infinite questions exist. Nevertheless, Keyes and Galea stress two fundamental types of research questions which have important implications selecting an appropriate study design.  

These are:

1. Questions whose goal is accurate estimation of population parameters

Examples:

• What proportion of high school students smoke? Or use drugs?
• What is the frequency of death from coronary artery disease among black and white males and females, and how have those rate changed over the past 20 years?

Questions like these require samples that are representative of the population being studied, that is comparable to the population in their characteristics (and they require adequate sample size in order to minimize sampling error).

2. Questions whose goal is to identify and quantify exposures that have causal effects on health outcomes.

Examples:

• Does use of cell phones cause cancer?
• Do "brain exercises prevent cognitive decline with advancing age?
• Do childhood vaccinations cause autism?

Questions like these also require an adequate sample size to precisely assess the magnitude of an effect, but they differ from questions aimed at parameter estimation in that that they require making comparisons, e.g., comparing risk between exposed and non-exposed persons. When trying to answer questions like these regarding etiology, it is not so important that the samples be representative of the overall population, but for accurate assessment of the effect the groups being compared must be comparable to each other with respect to other factors that affect the outcome.

Fundamental Study Designs for Both Representative and Purposive Studies

Keyes and Galea identify three fundamental approaches to study design that can be applied regardless of whether one's goal is to take representative samples to estimate population parameters or to take purposive samples in order to determine whether a given exposure or factor causes one or more health outcomes.

1. One can study the sample at a particular point in time.
2. One can follow the sample forward in time to compare the frequency of health indicators among two or more exposure groups.
3. One can examine the retrospective exposure history of a sample.

The second option will only be utilized in analytical studies, which will be covered in a separate module, but the first two options will be seen in the next section describing several types of descritive studies.

Categories of Descriptive Epidemiology

Case Reports

A case report is a detailed description of disease occurrence in a single person. Unusual features of the case may suggest a new hypothesis about the causes or mechanisms of disease.

Example: Acquired Immunodeficiency in an Infant; Possible Transmission by Means of Blood Products

Link to article by Ammann AJ et al: Acquired immunodeficiency in an infant: possible transmission by means of blood products. The Lancet 1:956-958, 1983.

In April 1983 it had not yet been shown that AIDS could be transmitted by blood or blood products. An infant born with Rh incompatibility; required blood products from 18 donors over 8 weeks and subsequently developed unusual recurrent infections with opportunistic agents such as Candida. The infant's T cell count was low, suggesting AIDS. There was no family history of immunodeficiency, but one of the blood donors was found to have died of AIDS. This led the investigators to hypothesize that AIDS could be transmitted by blood transfusion.

Example: Survival after Treatment of Rabies with Induction of Coma.

Link to article by Willoughby R, Jr., et al: N Engl J Med 2005;352:2508-14.

Rabies is almost uniformly fatal once it develops. As of 2005 there had been only four survivors, each of whom received rabies prophylaxis after the bite, but before symptoms developed. Willoughby et al. reported on a 15 year-old girl who rescued and released a bat that had struck an interior window. The bat bit her left index finger. The wound was washed with peroxide, but medical attention was not sought, and no rabies prophylaxis was administered. One month later she began to experience progressive neurological symptoms that were eventually diagnosed as rabies. The mainstay of her treatment was medically induced coma. Eight days later blood tests demonstrated that she had begun to develop an immune response to the rabies virus. Eventually the coma was reversed, and the patient gradually regained consciousness. She had severe neurological deficits, but gradually improved. She was discharged to her home after 76 days. Five months after her initial hospitalization, she was alert and communicative, but had persistent slurred speech and an unsteady gait.

The report by Willoughby et al. is an example of a case report – a detailed description of a single subject. The report is important because it demonstrates that it is possible for victims of rabies to survive, even without post-exposure prophylaxis. However, we have no idea how effective this treatment might be.

Case Series

A case series is a report on the characteristics of a group of subjects who all have a particular disease or condition. Common features among the group may suggest hypotheses about disease causation. Note that the "series" may be small (as in the example below) or it may be large (hundreds or thousands of "cases"). However, the chief limitation is that there is no comparison group. Consequently, common features may suggest hypotheses, but these need to be tested with some sort of analytical study before an association can be accepted as valid.

Example: Pneumocystis carinii pneumonia and mucosal candidiasis in previously healthy homosexual men: evidence of a new acquired cellular immunodeficiency.

Link to article by Gottlieb MS, et al: N Engl J Med 1981;305:1425-1431.

In 1980 –1981 four previously healthy young men were diagnosed with Pneumocystis carinii pneumonia, an unusual "opportunistic" infection that had only been seen in immune compromised people with hereditary disorders or in people with immune compromise due to chemotherapy. The medical histories didn't suggest any preexisting immunodeficiency, but all had decreased immune responses and low T cell counts. These unusual infections suggested the possibility of a previously unknown disease.  It was noted that all four men were sexually active homosexuals, and in the case series which was published in the New England Journal of Medicine the authors speculated that the immune dysfunction was due to a sexually transmitted infectious agent.

This was an extraordinarily important case series (a detailed description of characteristics of a series of people who all have the same disease) that suggested that this new syndrome was associated with sexual activity in male homosexuals. Alerting the medical establishment and proposing a hypothesis was an important milestone in the AIDS epidemic, however, the association could not be securely established based on this small case series. It was not known how many other individuals might be suffering from this new syndrome. It was also not known what the prevalence of homosexuality might be in others with this syndrome or how this might compare to the overall prevalence of homosexuality in the population that gave rise to the cases. As a result, this case series could not securely establish a valid association. Nevertheless, it laid the ground work for subsequent case-control studies and cohort studies (analytic studies) that did establish the risk factors for this disease.

Example: Oral Contraceptives and Hepatocellular Carcinoma?

There had been a number of case reports of liver cancers in young women taking oral contraceptives. A study was undertaken by contacting all of the cancer registries collaborating with the American College of Surgeons. The investigators wanted to collect information on as many of these rare liver tumors as possible across the US.  

Table - Oral Contraceptive Use Among Women Who Developed Liver Cancer

What conclusions can you draw from these data regarding a possible increased risk of liver cancer in woman taking oral contraceptives? Think about it before you look at the answer.

 Answer

Video Summary: Case Reports and Case Series (6:59)

Cross-Sectional Surveys

Cross-sectional surveys assess the prevalence of disease and the prevalence of risk factors at the same point in time and provide a "snapshot" of diseases and risk factors simultaneously in a defined population. For example, US government agencies periodically send out large surveys to random samples of the US population, asking about health status and risk factors and behaviors at that point in time. The Health Interview Survey (HIS) and the National Health and Nutrition Examination Survey (NHANES) are good examples.

The health questionnaires you are asked to fill out when you go to a new physician or being processed for a new job, or prior to entry into military service are similar to cross-sectional surveys in that they ask about the health problems that you have (heart disease? diabetes? asthma?) and your current behaviors and risk factors (e.g., How old are you? Do you smoke? What is your occupation?).

Cross-sectional surveys ask people their current status with respect to both exposures and diseases. This results in two main disadvantages.

1. The temporal relationship between exposure and disease outcomes can be unclear, i.e., which came first.
2. Cross-sectional studies tend to identify prevalent cases of long duration, since people who die quickly or recover quickly or who are no longer employed in a particular occupation are less likely to be identified.

Consider the following example in which a survey was conducted among white male farm workers. The survey asked many questions, but among them were the questions: "Have you been told you have coronary heart disease (CHD)?" And "How would you classify your level of physical activity?" The table below summarizes the findings. 

Table - Current Coronary Heart Disease Among Male Farm Workers

Note that the investigators did not follow these subjects over a period of time, so they did not assess the "incidence" of heart disease. Instead, they asked the subjects questions designed to determine the prevalence of heart disease, i.e., the proportion of the study population that had heart disease at this particular point in time. When they divided the sample into physically active and inactive farmers and computed the prevalence of heart disease in each of these, they found that CHD was much more prevalent among the inactive farmers. However, this was a cross-sectional study that related the prevalence of disease to the prevalence of activity at a point in time. They did not follow subjects over time to track the development of heart disease (i.e., the incidence). Consequently, the temporal relationship between the risk factor of interest (physical inactivity) and the outcome (CHD) is unclear. Had the farmers been physically active prior to developing CHD? Or, did they begin to limit their physical activity after they developed CHD? Consequently physical inactivity could have been either a cause of heart disease, or it could have been a consequence of CHD.

Large cross-sectional surveys are important for monitoring health status and health care needs of the population over time, and they are sometimes useful for suggesting possible associations between risk factors and diseases. However, the temporal relationship between the risk factor and disease is frequently unclear. Under these circumstances, they can generate hypotheses, but these associations need to be tested by appropriate analytical studies.

However, note that under some circumstances, the temporal relationship is clear on a cross-sectional survey. For example, if one conducted a survey of salaries of male and female professors to see if gender was associated with salary inequities, we could regard this as an analytical study, because it is clear that gender was established long before salary level. In this situation the temporal relationship between the "exposure" of interest (gender) and outcome (salary paid) is clear; we know that gender was established before the salary was negotiated. So, in a sense cross-sectional studies (and ecological studies can be thought of as an intermediate category between descriptive and analytic studies.

Video Summary on Cross-Sectional Surveys (8:25)

Ecological Studies (Correlational Studies)

These studies are distinguished by the fact that the unit of observation is not a person; rather it is an entire population or group. In essence, these studies examine the correlation between the average exposure in various populations with the overall frequency of disease within the populations.

In the study below investigators used commerce data to compute the overall consumption of meat by various nations. They then calculated the average (per capita) meat consumption per person by dividing total national meat consumption by the number of people in a given country. There is a clear linear trend; countries with the lowest meat consumption have the lowest rates of colon cancer, and the colon cancer rate among these countries progressively increases as meat consumption increases.

Note that in reality, people's meat consumption probably varied widely within nations, and the exposure that was calculated was an average that assumes that everyone ate the average amount of meat. This average exposure was then correlated with the overall disease frequency in each country. The example here suggests that the frequency of colon cancer increases as meat consumption increases. The characteristic of ecological studies that is most striking is that there is no information about individual people. If the data were summarized in a spread sheet, you would not see individual level data; you would see records with data on average exposure in multiple groups.

Morgenstern notes that, "Individual­ level variables are properties of individuals, and ecologic variables are properties of groups. To be more specific, ecologic measures may be classified into three types:

1. Aggregate measures are summaries (e.g. means or proportions) of observations derived from individuals in each group (e.g. the proportion of smokers or median family income).
2. Environmental measures are physical characteristics of the place in which members of each group live or work (e.g. air-pollution level or hours of sunlight). Note that each environmental measure has an analogue at the individual level, and these individual exposures, or doses, usually vary among members of each group, though they may remain unmeasured.
3. Global measures are attributes of groups or places for which there is no distinct analogue at the individual level. Unlike aggregate and environmental measures (e.g. population density, level of social disorganization. or the existence of a specific law).

Morgenstern goes on to note: "Ecologic study designs may be classified on two dimensions: (a) whether the primary group is measured (exploratory vs analytic study); and (b) whether subjects are grouped by place (multiple-group study), by time (time-trend study), or by place and time (mixed study). Despite several practical advantages of ecologic studies, there are many methodologic problems that severely limit causal inference, including ecologic and cross-level bias, problems of confounder control, within-group misclassification, lack of adequate data, temporal ambiguity, collinearity, and migration across groups."

For a detailed review of ecologic studies see follow the link to an article by Morgenstern H: Ecologic Studies in Epidemiology: Concepts, Principles, and Methods. Annual Review of Public Health 1995;16:61-81.

To see an extraordinary example of an ecologic study, play the video below created by Hans Rosling. This is a magnificent example that examines the correlation between income and life expectancy in the countries of the world over time. It is also a terrific example of a creative, engaging, and powerful way to display a vast quantity of data.

Advantages of Ecological Studies:

1. The data required is frequently readily available. Commerce data can be used to estimate a population's total consumption of products (possible risk factors) such as meat, tobacco, fish, etc. So, these studies are quick & inexpensive.
2. The "correlation coefficient" or an "r" value provides a measure of how closely the observed data points conform to a straight line. Some authors say that the "r" value is a measure of the association between the risk factor and the disease, but this is incorrect. The slope of the line would be a measure of the strength of association.  (See the course spreadsheet "Epi_Tools. XLSX" for a worksheet that calculates correlation coefficients). The value of a correlation coefficient is from +1 (a perfect positive correlation) and –1 (a perfect negative correlation). See the tabbed activity below for examples.

Limitations of Ecological Studies: It is important to bear in mind that the exposure in correlational studies is the average exposure for an entire population or group. This results in major limitations:

1. Since you don't have any information about the risk factor status or the outcome status of individual people, you can't directly link the risk factor to the disease, i.e., it is not clear that the people who ate the most meat were the ones who got colon cancer. This is sometimes referred to as "ecological bias" or the "ecological fallacy."
2. Another limitation is that there is no effective way of taking into account, or adjusting for, other factors that influence the outcome (confounding factors). As a result, an apparent correlation, or the lack of a correlation could be misleading. For example, one might find a strong correlation between the average number of hours of TV viewing & the rate of coronary artery disease among different countries. However, this doesn't necessarily mean that TV per se is a risk factor for CAD. There may be a number of other differences between the populations that are associated with higher rates of TV viewing: e.g., greater industrialization, less exercise, greater availability of processed foods and saturated fat, and so forth. And conversely, the lack of a correlation doesn't necessarily imply that there is no association.
3. Since the exposure levels represent average exposure in a large number of people, correlational studies can mask more complicated relationships, as illustrated below.

When a correlational study compared per capita alcohol consumption to death rates from coronary heart disease in different countries, it appeared that there was a fairly striking negative correlation.

However, a meta-analysis of prospective cohort studies which determined mortality rates in subjects for whom they had estimates of individual alcohol consumption, showed that there was actually a "J" shaped relationship. The people who drank the most actually had the highest mortality rates; moderate drinkers had the lowest mortality. This relationship was masked in the correlational study, because of the small percentage of people who have more than three drinks per day.

Adapted from: Di Castelnuovo A, Costanzo S, et al.: Alcohol Dosing and Total Mortality in Men and Women:  

An Updated Meta-analysis of 34 Prospective Studies. Arch Intern Med. 2006;166(22):2437-2445.

 Video Summary for Ecological Studies (7:48)

Summary & Self-Check

Descriptive studies are useful for:

• Monitoring and reporting on the health status and health related behaviors in populations
• Identifying emerging health problems
• Alerting us to potential threats from bioterrorism
• Establishing public health priorities for a population
• Evaluating the effectiveness of intervention programs and
• Exploring potential associations between "risk factors" and health outcomes in order to generate hypotheses about the determinants of disease.

Other Resources

1. University of North Carolina (UNC) -Torok M and Anderson M: "Focus on Field Epidemiology: Volume 5; Issue 5:Introduction to Public Health Surveillance."
2. University of North Carolina (UNC) - Anderson M: "Focus on Field Epidemiology: Volume 5; Issue 6: Public Health Surveillance Systems".
3. Trifonov V, Khiabanian H, Rabadan R: Geographic Dependence, Surveillance, and Origins of the 2009 Influenza A (H1N1) Virus. Perspective article in: N. Engl. J. Med. 2009;361(2):115-119.  
4. Scallan E, Hoekstra RM, Angulo FJ, et al. Foodborne Illness Acquired in the United States - Major Pathogens. Emerging Infectious Diseases 2011;17(1):7-15. [Volume 17, Number 1, January 2011, pages 7-15]
5. Marsden-Haug N, Foster VB, Gould PL, Elbert E, Wang H, Pavlin JA. Code-based syndromic surveillance for influenzalike illness by International Classification of Diseases, ninth revision. Emerg Infect Dis, Feb. 2007;13(2):207-216.

What is the study of the distribution of disease within populations?

What is the study of disease in a population called?

What is the study of the causes distribution and control of diseases in populations multiple choice question?

What kind of epidemiology is the study of the occurrence and distribution of disease?