An observational study is a type of study where the researcher can observe but does not administer any treatment. Therefore, whatever would normally happen, the researcher has to allow it to happen.
Researchers can't change anything about the people or subjects they are studying. The researcher can record the variables of interest, but again, can't affect the study. People have to be allowed to do whatever it is they were going to do without interruption.
There are two types of observational studies:
Retrospective Study, also known as a case-control study. Researchers look to the past to see what has already happened.
It can be similar to a matched-pair design in an experiment, but in this case, the researchers are not giving a treatment or doing anything to affect the people.
EXAMPLEIn a study, suppose you take a pair of participants, who are similar across most variables except for one major difference -- one participant has a disease, "the case", and one participant who does not have a disease, "the control". Because the participants are so similar, you are focusing on just that disease and seeing how it affects the participants or what causes the disease.
Prospective Study, also known as a longitudinal study, occurs over a long period of time. It observes the same set of people and follows the same variables over that chunk of time. It can be as many as several decades. While this type of study is not quick to do, it provides a lot of data and many different researchers can use this information in a variety of ways.
EXAMPLEThe Framingham Heart Study started in 1948 and is still going on today. 5,209 healthy adults from Framingham enrolled in this study. Researchers collected a variety of information about the subjects, including social networks, eating habits, exercise habits, and several markers for heart health.
Source: This work is adapted from sophia author jonathan osters.