# Odds ratios are inappropriate for a cross-sectional or cohort study

I'm aware of the issues regarding the use of relative risk in a case control study. But a colleague recently told me that odds ratios are inappropriate for cross-sectional and cohort studies but can't seem to elaborate further. I think he's just assuming that since relative risk is used in cohort studies rather than case controls, odds ratios must be used for case controls and not cross-sectional studies or cohorts. However, I can't find very much information of the appropriateness of odds ratios in these contexts. Can anyone point me in the right direction? Thanks!

Quick reference can be made to Bhopal's Concepts of Epidemiology 2nd edition (Ch. 9).

Odds ratios calculations are possible and valid in cohort, case-control and cross-sectional designs, but the OR is often not the estimate that is desired or is less efficient than alternatives.

1) Cross-sectional studies often want to estimate prevalence, so a relative measure such as OR wouldn't make sense if prevalence is your goal. It is certainly possible to estimate the OR though.

2) Cohort studies will have information about person-time at risk, and so the desired outcomes are often incidence rates, population attributable risk, or risk ratios. The odds ratio estimate for rare outcomes will approximately estimate the risk ratio in this design, but it makes more sense to compute the risk ratio directly.

• Excellent answer. The main flaw with the odds ratio is that it is misinterpreted almost all of the time. The bias of the "rare event" approximation of the OR to the RR is usually ignored, but in most cases it is rather substantial. Lastly I would add that, while we cannot estimate risk ratios from cross-sectional studies, the prevalence ratio is a meaningful association measure that is estimated in much the same way. Feb 1, 2018 at 14:46
• Thanks for that answer. This conversation came up in the context of a cross-sectional study examining the predictors of a binary behavior.
– TPM
Feb 1, 2018 at 20:10

Cohort studies report hazard ratio or relative risk. Case-control studies report odds ratio. Cross-sectional studies report prevalence odds ratio or prevalence ratio.

Base on 2X2 table, prevalence odds ratio and odds ratio share the same formula. prevalence ratio and relative risk share the same formula.