ECOLOGICAL STUDIES

Ecological epidemiological studies are observational descriptive studies that investigate the rate of a disease in relation to a disease factor demonstrated on a population level rather than at individual levels. This type of study can also be called correlational epidemiological study or cluster investigations, and it examines a disease using a group of people or the whole population without looking at it from an individual point of view.

Ecological studies are very useful for the generation of new hypothesis for a particular disease. In this type of study, a population or groups for correlational study are usually identified according to their occupation, place of residence, time, place of birth and other geographic and socioeconomic status.

The unit of study in an ecological (correlational or aggregated) study is population group and not individuals (as is the case in order observational studies). In ecological studies it is difficult to connect the exposure of an individual to his or her outcome and, this places some limitations on this type of study in terms of extrapolating the result of a group or population study to individuals in the population.

Most correlational studies make use of some routinely collected data (or data from previous studies) such as information from people’s lifestyle, data from health-related surveillance programmes, environmental data and other demographical data.

Correlational studies can also be undertaken by using already collected or available data to compare the occurrences of a disease between different groups during the same period of time or in the same population at different series of time. Thus, associations of a disease at the group/population levels cannot be inferred or correlated to individual levels. 

MERITS OF ECOLOGICAL STUDIES

• Ecological studies can be done quickly and inexpensively.

• They rely on pre-existing information, thus it is the most popular type of study design amongst epidemiologists.

• They achieve a wider range of exposure levels than could be anticipated from an individual type of study since ecological studies looks at a disease from a population or group level rather than from a typical individual point of view.

• They are useful for investigating cases of transmission of infectious diseases since they take into cognizance ecologic effects such as demography.

• They are effective in monitoring the outcome of public health interventional measures such as disease screening and health education programmes in a population.

• Correlational studies are the best type of study for investigating diseases or exposures which are easier to evaluate at a group level than at an individual level.

DEMERITS OF ECOLOGICAL STUDIES

• Ecological studies lack basic individual-level information, and this is leads to a phenomenon called ecological bias or fallacy. This is because a relationship observed at a group level cannot represent an association at the individual level.

An ecological fallacy or bias occurs in an epidemiological study when wrong conclusions are drawn on the basis of ecological data. The fallacy in a correlational study occurs because the relationship observed by the researcher between variables at the group level of the study does not represent to an extent the connection that exists at the individual level.

• The results of this study are often difficult to interpret.

• They are limited because the study relies on data collected previously for other purposes.

• The connection between exposure and effect (or disease) at the individual level cannot be made since the unit of study in ecological study is a group. Thus the result of such studies cannot be extrapolated to individuals since their results refer to groups of people.

References

Aschengrau A and Seage G.R (2013). Essentials of Epidemiology in Public Health. Third edition. Jones and Bartleh Learning,

Aschengrau, A., & G. R. Seage III. (2009). Essentials of Epidemiology in Public Health.  Boston:  Jones and Bartlett Publishers.

Bonita R., Beaglehole R., Kjellström T (2006). Basic epidemiology.  2^nd edition. World Health Organization. Pp. 1-226.

Porta M (2008). A dictionary of epidemiology. 5^th edition. New York: Oxford University Press.

Rothman K.J and Greenland S (1998). Modern epidemiology, 2^nd edition. Philadelphia: Lippincott-Raven. 

Rothman K.J, Greenland S and Lash T.L (2011). Modern Epidemiology. Third edition. Lippincott Williams and Wilkins, Philadelphia, PA, USA.