How to calculate an odds ratio

odds ratios are the measure of association in a case control study. This video demonstrates the calculation of the OR

[fullnelsondnb]

We had a nice way of memorizing this, finding the ad/bc method somewhat difficult to remember at times, especially when lumping it with specificity and sensitivity, which were also taught with the ABCD box. Instead, we would say, "(BOTH x NONE)/(SOME x SOME)." where  BOTH = exposure and the disease, NONE =  no exposure and no disease, SOME = either exposure without the disease or disease without exposure (it doesn't change the outcome in the denominator since it's simple multiplication, for example 2 x 3 is = 3 x 2).  Can't go wrong. 

[hazelpresdee]

Brilliantly simple exposition of a concept I have found daunting.

[ProfFeinman]

The reason the cross-product works is that you are dividing the odds of having been a smoker 90/10 by the odds of not being a smoker 10/90 (it's a ratio). Division is multilication by the reciprocal which is that you do with the cross-product. and as comments say, any numbers will work. Very nice presentation.

[kdoptm]

Thank you for this!! It was posted 8 years ago but still helping students like me

[GraceAdeniyiOlamijulo]

It does help me, soooo helpful!!! A million thumbs up from me.
Thanks!
From Nigeria

[lennydog]

Such a simple way of explaining this! thank you

[b.r]

I don't know why my teachers like complex formulas when getting OR was this easy !! . Cross product works magic. Thanks

[elizabethannjoseph]

Great vid, and a way better explanation than my professor gave in class. Thanks a bunch for posting this vid :)

[laxmanbisht]

The explanation which you mentioned is that "If you had lung cancer, there is 81 times more likely to smoke if you did not have.". I find it difficult to accept. Would not it be another way around. If you smoke, there is 81 times more chance (odds/likely) to get lung cancer than if you do not smoke. Please correct me If I am wrong.

[drgarad]

You`re a legend in this game.
Thanks for the help.

[CelticYogi]

Terry, could you please explain the difference between, "If you have lung cancer, then you are 81 times more likely to have smoked" AND "if you smoke, you are 81 times more likely to get lung cancer." Or is there not a difference? I ask because the way you phrase it does not infer that smoking causes cancer, but that the behavior and disease are associated. However, there's an example in the text book that interprets the OR this way: "smokers are 1.6 times AS LIKELY to develop LC as nonsmokers." Any clarity? Thank you.

[ernuoc]

Hey Anne, I think you might be mixing up "Odds Ratio" (presented here), which is the mathematical inference drawn from case-control studies - "how much more likely were you exposed to the risk factor if you have the disease?", versus "Relative Risk", which is the mathematical inference drawn from cohort studies - "how much more likely would you have the disease if you are exposed to the risk factor?" The first study is retrospective with the latter being prospective

[MsBubblyshe]

thank you! it took me hours to study this on the book, only 3 minutes here. :)

[boblolo]

That is so uplifting, You could also used as an example the odds of finishing in an oven if you lived in the Warsaw ghetto.

[frank-yqcf]

Thank you, a nice way to understand OR.

[rosannaruiz]

Thanks for a short and to the point explanation! In a Epi class this helped me!

[chundak]

Your videos are nice and helpful. Keep up the good work

[islamkhalil]

Your videos on OR and RR are helpful for my Epidemiology class. My professor sucks. Thanks

[UABEBMcourse]

Since this is a case control study you can only estimate "risk" of exposure

[VisswanathVenkataraman-qn]

Excellent...no nonsense video...right on the money!