I might be wrong, but I certainly thought case-control was a retrospective study. You look at people with the disease, and see what risk factors they had. Cross-sectional is the snapshot in time you speak of...
A cross-sectional will also get you a snapshot in time.
Case-control - take a bunch of people with a disease, take a bunch of people without the disease (in academia, this means the grad students down the hall), and apply some test to both groups. E.g. "Our new lung CA test had a sensitivity of 0.90 comparing between cases and controls." No causation, but correlation can be inferred from these results.
Cross-sectional - take a bunch of people, give them all surveys, a test, whatever, and analyze the results. You can say things like, "The people who scored highest on lifetime use of ectasy had the lowest fMRI results for areas 1, 2, and 5." There's no comparisons between groups, there's just how a group looks.
I will have to think about your mnemonic - I always get RR and OR mixed up.
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On cohorts - people are grouped by one common trait - birthday, a risk factor, activity, or being from Boston. It doesn't matter. Then you observe them for (usually) 5 years. You define your outcome measures (usually death or develpoment of the disease of interest) at the beginning. Think of the Framingham study.
I can't think of a way that you could do a retrospective case-control study. You could look at all of the charts of people admitted to a hospital over a given time and how their diagnoses and outcomes compared to another group's, but you might as well just call it a retrospective cohort study at that point.
EDIT: OK, I guess I got some things confused. Taking a
look at this, I need to brush up on this subject myself.