RE: BMJ study question

["Rick Orthen" <rorthen@cecinc.com>]

Rich--Selectively quoting from

http://www.childrens-mercy.org/stats/journal/oddsratio.asp



"Both the odds ratio and the relative risk compare the likelihood of an

event between two groups. . . The odds ratio compares the relative odds

of death in each group. . . . The relative risk (sometimes called the

risk ratio) compares the probability of death in each group rather than

the odds.  



There are three issues here: The relative risk measures events in a way

that is interpretable and consistent with the way people really think.

The relative risk, though, cannot always be computed in a research

design. Also, the relative risk can sometimes lead to ambiguous and

confusing situations.



Both the odds ratio and the relative risk compare the relative

likelihood of an event occurring between two distinct groups. The

relative risk is easier to interpret and consistent with the general

intuition. Some designs, however, prevent the calculation of the

relative risk. Also there is some ambiguity as to which relative risk

you are comparing. When you are reading research that summarizes the

data using odds ratios, or relative risks, you need to be aware of the

limitations of both of these measures.



What does the odds ratio estimate in a case-control study? Pearce N. Int

J Epidemiol 1993:22(6);1189-92.  The use of the term 'odds ratio' in

reporting the findings of case-control studies is technically correct,

but is often misleading. The meaning of the odds ratio estimates

obtained in a case-control study differs according to whether controls

are selected from person-time at risk (the study base), persons at risk

(the base-population at risk at the beginning of follow-up), or

survivors (the population at risk at the end of follow-up). These three

methods of control selection correspond to estimating the rate ratio,

risk ratio, or the odds ratio respectively, by means of calculating the

odds ratio in the subjects actually studied. None of these estimation

procedures depends on any rare disease assumption. Where the rare

disease assumption is relevant is whether the effect which is estimated

(e.g. the odds ratio) is approximately equal to some other effect

measure of interest (e.g. the risk ratio or rate ratio) in the

underlying study base. To avoid confusion on this issue, authors should

be encouraged to not only specify the manner in which controls have been

selected (e.g. by density sampling) but also the corresponding effect

measure which is being estimated (e.g. the rate ratio) by the 'odds

ratio' which is obtained in a case-control analysis."



As usual with statistical interpretations, your mileage may vary.

 

Richard F. Orthen, CHMM

Senior Project Manager

Civil & Environmental Consultants, Inc.

Four Triangle Lane, Suite 200

Export, PA  15632-9255

724/327-5200, ext. 231

www.cecinc.com



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