Evans County Cardiovascular and Cerebrovascular Study

Background/Question:

The Evans County Study examined differences in incidence of cardiovascular disease across the entire population of Evans County, Georgia. The study is unique in that it was the only epidemiologic study at the time to compare differences in disease incidence between blacks and whites, and one of only two epidemiological studies at the time to examine the entire population of a community. The principal investigator sought to answer two questions: Does coronary heart disease truly occur more frequently in blacks than whites in Evans County, and if so, how extensive are these racial differences?

Methods:

Using a prospective cohort model, the investigators sought to examine the entire population of the county aged over 40 years and half the population aged between 15 and 39 years. They ultimately achieved a 92% response rate for a total study population of 3,102 residents. From 1960-1962, each participant was given an initial examination in which serum cholesterol, blood pressure, weight, height and the participant’s responses to a number of health behavior and medical history questions. A follow-up consisting of a similar battery of tests was completed from 1967-1969. For 90.9% of the study population, a physical similar to the preliminary one was completed. For another 7.9%, health status was ascertained by a phone survey of either subject or a family member. The total rate of follow-up was 98.8%. Factors assessed at the time of follow-up were systolic and diastolic blood pressure, serum cholesterol level, cigarette smoking, body weight, hematocrit value, ECG abnormalities and diet. Socioeconomic status was also assessed for whites, but not for blacks, as little variability was found in this factor among black subjects. [1]

Results:

A central finding of the study was that, across race and social class differences, factors associated with increased risk of coronary heart disease did so equally for blacks and whites whether the risk factors were considered singly or in combination. CHD incidence remained markedly higher for white Evans County residents. The only social grouping of white residents that had equally low incidence of CHD was white sharecroppers, which was attributed to higher rates of physical activity among sharecroppers when compared to whites of higher socioeconomic status. [1]

Conclusions/Discussion:

A strength of the study was the high response rate of the population in its entirety, one of only two of its kind to successfully examine an entire community. The publication of the initial study results did not include statistical analysis. Hames attributes his decision to avoid statistical analysis to the unique design of the study. He wrote, “…it certainly is not clear to us [the investigators] what differences such tests could make in our interpretations in the context of this study… the population over 40 does not constitute a sample of some universe. They are a universe.” [1] (HB)