• Prospective studies begin prior to the exposure and study the population over time.

An example is the Nurses Health Study in which thousands of nurses kept records of lifestyle factors that may have been related to disease. Overtime, some of the nurses developed specific diseases, while others didn't.

Epidemiologists then looked at the lifestyle data gathered to determine whether there were any factors that were different among those who did versus did not develop the disease.

• Historical or retrospective studies look back in time for patterns of exposure that may have differed among the groups.

These studies look at groups of people who have or have not developed a disease and compare them. When these studies rely on health or occupational records they can be very useful. However, when they are based on subjects' memories, they may be less reliable.

 

Advantages:

Cancer doesn't just suddenly appear in a person. Cohort studies allow researchers to study people over the long period of time that it takes for cancer to develop. Cohort studies have an advantage over case-control studies because they ask people to:

	Keep track of what they're eating while they're still healthy, rather than waiting until they develop cancer and then asking what they ate in the past (pre-diagnosis). Many different types of cancer (or other diseases) can be studied using the same cohort.
Image courtesy of American Institute of Cancer Research

 

Disadvantages:

As in any kind of dietary investigation, cohort studies need to be extremely large and follow participants over a long period of time in order to pack real scientific and statistical punch.