all this should be explained in intro to epidemiology courses/textbooks
Im not sure if youre aware of the differences but here's a quiiiick summary
First, these three (cohort casecontrol crossectional) are all observational studies, which are convenient but do not have randomization, they are not the classic "scientific method" perfectly set up experiment, you are just observing what is happening to people and trying to make conncetions between exposures and diseases, as opposed to randomized clinical trials, where you design an "experiment" where you randomize some people into a group to get the exposure and others to get a placebo, randomized are best, but are often not feasible or not ethical (you cannot randomize people into a group that is required to smoke 3 packs a day)
cohort=prospective=follows a group or "cohort" forward in time, basically you have a group of people that you know are exposed to a potential pathogen or environment or whatever, and then you take a group of people that were not exposed, and then you follow them both over a period of time and compare the rates of disease that develop between the two groups. note that in the beginning nobody had the disease. this is the best method for observational studies, but can be expensive and hard for rare diseases because if its too rare maybe nobody in the study will ever develop the disease
case control= you start with people with the disease "cases", find a group of similar people without the disease "controls" and then look back into the past (retrospective) to see what the cases were exposed to that the controls were not, not as good as cohort because your asking people to remember into the past what they might have been exposed to, people forget, esp. the controls forget because theyre not thinking about why they might be sick, but case controls are convenient for rare diseases because you start with people wo have the disease youre not sitting around waiting for people to get a rare disease
cross sectional= a quick kinda shotgun approach, similar to case control, except you give people a questionnaire that asks about disease status and previous exposures, so you determine disease status and potentially dangerous exposures simultaneously, and see if one common thing keeps popping up on the lists of exposures in people that also check the box for having the disease. super convenient but lots of potential for bias
