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anwer_khur

Re: use of odds ratio to establish association in cross sectional study

anwer_khurshid@yahoo.com

ibrahim_ap

Re: Re: Re: use of odds ratio to establish association in cross sectional study

Dear Anwar Sb, Can i got your email address
Muhammad Ibrahim

ibrahim_ap98@yahoo.com

anwer_khur

Re: Re: use of odds ratio to establish association in cross sectional study

The answer is yes. There are statistical methods (based on heavy mathematics) to estimate the missing freuencies i.e. a, b, c, d or the marginal frequencies (column and row toatals) ie. a+b, a+c ...

Risk ratio (RR) and odds ratio (OR) are two measures for comparing risks; the former relates the chance or risk of developing outcome for exposed versus unexposed groups, the latter relates their odds. Because odds are not ordinarly used in common language as chance or probability or risk, many people find the concept of OR a bit more difficult to interpret than that of RR.

A word of caution:
You have written OR=ac/bd, which is not the form we statisticians use. Let me explain;
Our sample data in a prospective study looks like as
Exposed Unexposed Total
Cases a b a+b
Noncase c d c+d
Total a+c b+d n

for which OR=ad/bc and not what you have written. To find RR we can use the fact
RR=[(a/a+c)/(b/(b+d)]=(ab+ad)/(ab+ac)

For details you may consult my book
Sahai, H. and Khurshid, A.(1995). Statistics in Epidemiology: Methods, Techniques and Applications. CRC Press, New York.

ibrahim_ap

Re: Re: use of odds ratio to establish association in cross sectional study

Yes you can use this formula for the calculation of RR. (ac+ad)/(ac+bc). for any assistance call 0300 4668681

Grossville

Re: use of odds ratio to establish association in cross sectional study

Not a comment, but a question: Is there a specific formula to convert OR into PR? rather than ac/bd to a/(a+b) : c / (c+d)? Just in case there is not specified the number of a,b,c,d? Thank you