Cohort studies are prospective in nature.
You suspect that, for example:
So you set out to examine some representative cases very closely to see if you are correct. As always, it can be exciting to think that you may have noticed something that no one else has noticed, and you want to proceed according to certain rules to insure that you remain honest and prudent.
Here is a simple schematic of the Cohort study:
It is important to note that what makes the Cohort design "prospective" in nature is that you are working from suspected cause to effect (or outcome). It can be a concurrent study, meaning that you start collecting data now; non-current, typical of a chart review or review of other records, or; a combination of the two.
Case-control studies are retrospective in nature.
You have patients that are sick with a new disease. There are others from the same area that are not infected. You begin to backtrack to try to find the source or common element of the infection.
It can be very exciting to think that we may have noticed something that no one else has noticed. This is where rules (research design) come into play to keep us honest and prudent.
Remember that in the case-control schematic:
It is important to note here what is meant by "retrospective". All this means is that you are working from outcome to suspected cause. You can start collecting data, or you can do a chart review, or you can do a combination. What makes it "retrospective" is that you are working from effect to cause, not vice-versa.