Cohort studies are conducted based on some common characteristics among the population. Cohort studies are conducted just like longitudinal study except that the target population in the cohort studies share some common characteristics. The common characteristic can be any factor like age, employment, marriage, education year, fertility behavior or other demographics. A cohort study can be conducted like a cross-sectional study or longitudinal study. If a cohort study is conducted like a cross-sectional study the cross-section of a population is studied once which share same cohort characteristics. If a cohort study is conducted like a longitudinal study the target population is studied more than once and the sample in the target population can be different each time but they share same cohort characteristics.
A cohort study has a great many applications in the fields of medicine, social science, psychology, marketing, merchandising, business, environmental sciences etc. The cohort studies can be a retrospective study or a prospective study, therefore, they have greater significance.
Examples
Some examples of cohort studies can be as follows;
A cohort study to know the impact of latest hepatitis A vaccine impact on the health of a group of kids.

To study the effects of environmental science pollution on the current population of Virginia, keeping in mind the respiratory health issues.

A study on the impact of fast food consumption on obesity among the teenage girls lived in McLean from 2000-2005.

To study the prevalence of intestinal cancer among people eating veg and non-veg diet from 2000-2010.

To study the effects of smoking on lung cancer over a period of time.

Retrospective cohort studies
Retrospective cohort studies are done to know the prevalence and reasons of a problem or situation in the past. They are very useful because the output of these studies can be used to predict future. The only problem with these studies is that the researcher cannot maintain the validity of the research completely. The researcher has to compromise on the data that is available and he cannot make sure that the extraneous factors had no impact on the particular situation or problem.

In the field of medical science, retrospective cohort studies provide valuable information at low costs. The validity of these studies is however questionable because the researcher cannot obtain first-hand information.

Prospective cohort studies
A prospective cohort study studies the population in the present situation and makes assumptions for the future. The researcher takes pre-test evaluations of the audience and then another test is taken after the application of the program or treatment to the audience. The researcher makes sure that the extraneous factors are kept in control. These studies are more valid then the retrospective cohort studies because the researcher has more control throughout the study. In the field of medical laboratory science and psychology, some studies take several years to complete but the information they provide is valuable to the masses. Such studies are usually conducted on a large sample to maintain the validity of the study.