# Thread: Help with interactions in regression

1. ## Help with interactions in regression

Hi
I am doing a regression analysis, looking at the effect of 2 drugs (A or B) in a population where we stratified individuals by ethnic group:
Estimates for European Population

Effect OddsRatioEst LowerCL UpperCL ProbChiSq
Age 1.005 0.973 1.037 0.7729
Drug 1.862 1.229 2.821 0.0033
Sex 1.469 0.925 2.334 0.1033
Weight 1.015 1.001 1.028 0.0346
eGFR 1.006 0.995 1.016 0.2967

________________________________________
Estimates for Asian Population

Obs Effect OddsRatioEst LowerCL UpperCL ProbChiSq
1 Age 1.047 0.994 1.103 0.0839
2 Drug 0.719 0.217 2.382 0.5892
3 Sex 1.932 0.445 8.384 0.3795
4 Weight 0.987 0.954 1.020 0.4357
5 eGFR 1.014 0.991 1.036 0.2300

As you can notice, the effect of the drug A is significantly better than B in the European, but not in the Asian population. One could interpret this as the effect of the drug being modified according to the study subpopulation, and hence expect the p value for interaction to be significant, but it is not at all!
________________________________________
Interactin term P value

ProbChiSq
0.1874

Any explanation please? Am I missing something

2. ## Re: Help with interactions in regression

The fact than an estimate is significant in one sub-population and not significant in another does not mean that the estimates are signficantly different. See: Andrew Gelman and Hal Stern (2006) "The Difference Between “Significant” and “Not Significant” is not Itself Statistically Significant" The American Statistician, 60(4):328-331. http://www.stat.columbia.edu/~gelman...ed/signif4.pdf

3. ## The Following User Says Thank You to maartenbuis For This Useful Post:

Bio_Stat (03-04-2015)

4. ## Re: Help with interactions in regression

In addition, the sample size (dunno why hardly no-one posts
this crucial information within his problem description) for Asians
seems to be very low; although the effect goes into opposite
direction compared to Europeans, interpretation of the significance
test for A/B in Asians, or of the non-significant interaction will be
difficult, due to low power.

With kind regards

K.

5. ## The Following User Says Thank You to Karabiner For This Useful Post:

Bio_Stat (03-04-2015)

6. ## Re: Help with interactions in regression

It might also be of interest to plot the probability of outcome in euro with drug a and then same thing for drug b. Then draw a line connecting the plots. Then on the same graph do the same thing with Asians probabilities.

See if lines cross. Also use the probabilities from the full model.

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