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Thread: Strange results in univariable linear regression when groups are stratified

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    Strange results in univariable linear regression when groups are stratified




    Dear Forum users,

    I am stuck with some unusual results and would appreciate some ideas as to why/how this happened. Iím testing the relationship between two biological continuous variables in a large cohort study (n>5000), using simple univariable linear regression on Stata. I stratified the analysis for male and females, and then repeated with both groups together. For males, the beta & 95%CI is -1.5 (-2.0, -1.0, p<0.001), for females, its -1.8 (-2.3, -1.4, p<0.001), but for both combined, its 0.09 (-0.16, 0.34, p=0.47). I canít explain how the results for M and F separately show a significant negative association, but for both combined, its not significant.

    Strange results again using the same outcome variable, but a different dependent variable in the same study. For males, itís -0.18 (-0.69, 0.33, p=0.48), for females itís -0.91 (-1.38, -0.43, p<0.001), but for both combined, its 2.02 (1.77, 2.23, p<0.001). So males is not significant, female itís significantly NEGATIVE, but for both, itís significantly POSITIVE. The combined group has 58% female.

    Iíve rechecked the results many times and there are no mistakes that I can see.

    Thanks for your help.

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    Re: Strange results in univariable linear regression when groups are stratified


    Read about Simpson's paradox.

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