Descriptive studies, sometimes known as geographical or ecological studies, can be used to demonstrate patterns of disease and associated factors in a population. The units of study are populations or groups.

Examples include:

• Correlating population disease rates with factors of interest such as health care use
• Demonstrating changes in mortality over time
• Comparing the prevalence of a disease between different regions at a single point in time

This type of study can be used to generate hypotheses of possible causes or determinants of disease, for example if broad geographical differences are seen, but cannot be used to prove this or explore associations in any depth. Further observational or even interventional studies are needed to do this.

Descriptive studies usually make use of routinely collected health information, such as HES data or infectious disease notifications, so their principal advantage is that they are cheap and quick to complete. However, the appropriate data may not be readily available and in these circumstances it is necessary to carry out special surveys to collect the raw material necessary for the study.

Descriptive studies are often used for health service planning, for example assessing the burden of a disease in a community, but other uses also include:

• Investigation of possible correlations
• Surveillance of health states
• Studies of disease clustering
• Monitoring the effectiveness of health interventions

Ecological fallacy

Data from descriptive or ecological studies describe the characteristics of a group. The ecological fallacy is an error in the interpretation of statistical data in an ecological study that results when concludions are made about individuals from the aggregated data inappropriately. 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 an individual plucked from the group.

Reasons for the ecological fallacy include:

• 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
• Data used in descriptive studies were usually collected for other purposes originally
• Use of average exposure levels may mask more complicated relationships with the disease
• Inability to control for confounding

Strengths of ecological studies

• Cheap and simple to conduct.
• Utilize routinely collected health statistics.
• Exposure data often only available at area level.
• Differences in exposure between areas may be bigger than at the individual level.
• Utilize geographical information systems to examine spatial framework of disease and exposure.
• Generate hypotheses to examine at the individual level.

Weaknesses of ecological studies

• Measures of exposure are only a proxy based on the average in the population. Caution needed when applying grouped results to the individual level (ecological fallacy).
• 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.
• Potential for systematic differences between areas in the measurement of exposures.
• Lack of available data on confounding factors.

Descriptive studies are addressed by the following MFPH Part A past paper questions:

• January 2002 – Paper IA, Question 1
• May 2005 – Paper IA, Question 1

© Helen Barratt, Maria Kirwan 2009