When the study began, the investigators settled on a target sample size of 6,000 people between the ages of 30 and 59. People in this age window more commonly develop CVD, yet a sample in this range would also include a significant number not already experiencing symptoms. Since the idea was to see why and how CVD develops over time, these two factors were key. Participants had to show up every two years to provide updated medical histories and have physical exams and lab tests done. Researchers hypothesized that there wasn't one cause of CVD, but many, so the medical histories and exams would be crucial to finding out what these causes were.
Two-thirds of the families in Framingham were approached to be part of the sample. Local civic committees and clubs personally contacted people to encourage them to participate. These efforts brought 4,469 people in for examinations, which wasn't enough. The study was expanded to include volunteer participants who hadn't been contacted initially. Another 740 of those volunteers were added to what's known as the "Framingham Cohort," numbering 5,209 men and women between the ages of 28 to 62.
Two issues resulted from this method: The sample group was not completely random, as the investigators had hoped, and the participant group was healthier than the general population. The researchers ended up adding people who had CVD to the Framingham Cohort [source: Framingham Heart Study].
In 1971, the study expanded to include offspring of the original cohort after President Nixon worked his research-extending magic. This "Offspring Cohort," comprised 5,124 people, and included spouses. This new group served a dual purpose. First, adding the children allowed researchers to look for evidence of familial clustering of CVD, while the extra layer of spouses allowed for examination of unrelated young adults [source: Mahmood].
In 1994, the First Omni Cohort (one of an eventual three) was established to address the increasingly diverse population of Framingham. The first of these cohorts was made up of 507 people with Native American, African-American, Hispanic, Indian, Asian and Pacific Islander roots who were local to Framingham or surrounding areas.
The study expanded yet again with the establishment of the Third Generation Cohort (Gen III), which began in 2002 and is ongoing and expected to close in 2019. It is currently following 4,095 people aged 19 to 79 at the study's onset, who have at least one parent enrolled already participating in the Offspring cohort. Yet another group added to the mix is the New Offspring Spouse Cohort, composed of spouses from the Offspring Cohort who never signed on before, for whatever reason, and who have two or more offspring taking part in Gen III.
Over time, the questions asked in the medical histories and the tests administered changed as the researchers learned more about the causes of CVD.