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Fifth Dimension Table of Contents

Epidemiology Introduction:

M. J. McKeown, MD, FACOG, FACS

Ecological Study


This is the basic science of public health medicine. This discipline involves studying the distribution and determinants of the frequency of disease in a population.

These studies may be descriptive or analytical. Descriptive studies are those that investigate the distribution of a disease in a population. Analytical studies are those that look at the determinant factors (causes) of a disease in a population.

Each of these types of studies attempts to determine certain factors describing a disease in the population studied.


The frequency of a disease in a population is described by the rate of that disease.

Rate is the number of cases of a disease (numerator) divided by the number of those in the population at risk (denominator).

Prevalence rate is the proportion of the population who have the disease or condition.

Incidence rate is the number of new cases of the disease in the population arising during a specific period of time.

The general description of a disease in a population looks at how much of the disease or condition exists in a time defined as the duration of the disease.
      prevalence = incidence x disease duration

Case fatality rate describes the number expected to die from the disease or condition.
      case fatality rate = ­number of deaths from the condition
          number of cases of the condition

If the condition is always fatal then the case fatality rate is 100% and the mortality rateis then equal to the number of cases that occur (incidence rate).

Ecological Study

If a study involves the investigation of the characteristics of a disease or condition in a whole population it is considered an ecological study. This is in contrast to studies that investigates the characteristics of a disease or condition in an individual.

Incidence is the number of new cases of a disease or condition over a defined unit of time in a defined population. This is the number per unit population per unit of time.

Prevalence is the existing number of people with a disease or condition in a population. This is the number of cases or conditions per unit population.

An ecological study has the advantages of being less expensive, easy to perform using routinely collected data, providing new hypotheses about the causes of a disease or condition, and providing new potential risk factors.

However an ecological study has disadvantages. The ecological fallacy is believing that the average value of the characteristic being studied and the average incidence/prevalence of a disease or condition applies to all the individuals in a population. An example would be to relate the dietary fat consumption of the average individual to the incidence or prevalence of cancer of the colon in the population at large. This misuse of ecologic data is not infrequent by political persuasions on either side of a question involving the health of a population in general.

An ecologic study is subject to error due to the willingness of a particular population to seek help for a given condition and thus come to be included in the data for that disease or condition.

An ecologic study is subject to errors in the quality of the recording system.

An ecologic study is subject to errors in defining diagnostic criteria used to enter data about a given disease or condition into any such recording system.

Ecologic studies are particularly used when a society attempts to evaluate and treat or eliminate a disease or a condition causing a disease whose existence is a significant economic threat to the society. Current most common studies focus on heart and blood vessel disease and on cancer.

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