The design, applications, strengths and weaknesses of descriptive studies and ecological studies

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Descriptive studies are frequently the first step into a new line of enquiry, and as such have an important role in medical research, where their findings can prompt further study. Their function is to describe the “who, what, why, when, where” without regard to hypothesis, highlighting patterns of disease and associated factors.

Descriptive studies that examine individuals can take the form of case reports (a report of a single case of an unusual disease or association), case series (a description of several similar cases) and cross-sectional studies (see “Cross-sectional, analytical and intervention studies”).

Descriptive studies that examine populations, or groups, as the unit of observation are known as ecological studies. Ecological studies are particularly useful to conduct when individual-level data would either be difficult or impossible to collect, such as the effect of air pollution or of legislation. Examples of the use of ecological studies include:

Correlating population disease rates with factors of interest, such as healthcare use
Demonstrating changes in mortality over time (time series)
Comparing the prevalence of a disease between different regions at a single point in time (geographical studies)

Ecological studies often make use of routinely collected health information, such as hospital episode statistics in the UK or infectious disease notifications, so their principal advantage is that they are cheap and quick to complete. However, where appropriate information is not readily available it is necessary to carry out special surveys to collect the raw data necessary for the study.

Application

All forms of descriptive study can be used to generate hypotheses of possible causes or determinants of disease. These hypotheses can then be tested using further observational or interventional studies. Case reports can identify novel associations, such as the development of a rare benign liver cancer in a woman taking oral contraceptives¹. Case series are useful in identifying epidemics. For example, the presence of AIDS in North America was identified by the report of a cluster of homosexual men in Los Angeles with a similar clinical syndrome².

Ecological studies are a useful means of performing international comparisons and studying group-level effects (for example, the correlation between deaths rates from cardiovascular disease and cigarette sales per capita).

Strengths and Weaknesses

Descriptive (including ecological) studies are generally relatively quick, easy and cheap to conduct. Particular strengths of ecological studies include:

Exposure data often only available at area level.
Differences in exposure between areas may be bigger than at the individual level, and so are more easily examined.
Utilisation of geographical information systems to examine spatial framework of disease and exposure.

Weaknesses of case reports and case series are that they have no comparison (control) group, they cannot be tested for statistical associations, and they are especially prone to publication bias (especially where case reports/series describe the effectiveness of an intervention).

Limitations of ecological studies include:

Measures of exposure are only a proxy based on the average in the population. Caution is needed when applying grouped results to the individual level (ecological fallacy, below).
Potential for systematic differences between areas in recording disease frequency. For example there may be differences in disease coding and classification, diagnosis and completeness of reporting between different countries.
Potential for systematic differences between areas in the measurement of exposures.
Lack of available data on confounding factors.

Ecological fallacy

The ecological fallacy is an error in the interpretation of the results of an ecological study, where conclusions are inappropriately inferred about individuals from the results of aggregate data. The fallacy assumes that individual members of a group all have the average characteristics of the group as whole, when in fact any association observed between variables at the group level does not necessarily mean that the same association exists for any given individual selected from the group. For example, it has been observed that the number of televisions per capita is negatively associated with the rate of deaths from heart disease. However, it would be an ecological fallacy to infer that people who don’t own televisions die from heart disease ³. Indeed, in this scenario there are other potentially causative factors that could be common to both, such as reduced physical activity or a poorer diet associated with less affluent societies.

Reasons for the ecological fallacy include the following:

It is not possible to link exposure with disease in individuals - those with disease may not be the same people in the population who are exposed.
The data used may have originally been collected for other purposes.
Use of average exposure levels may mask more complicated relationships with the disease, such as the J-shaped relationship between alcohol consumption and heart disease.
Inability to control for confounding.

References

Schenken JR. Hepatocellular adenoma: relationship to oral contraceptives? JAMA 1976; 236: 559.
Anon. Pneumocystis pneumonia: Los Angeles. MMWR Morb Mortal Wkly Rep 1981; 30: 250–52.
Grimes DA, Schulz KF. Descriptive studies: what they can and cannot do. Lancet 2002;359:145-9.