Understanding probabel GxE interaction output

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Joined: Thu May 21, 2015 8:13 pm

Understanding probabel GxE interaction output

Postby agiri_genepi » Thu May 21, 2015 8:36 pm

Hello, I'm new to GenABEL and ProbABEL. I'm using ProbABEL at the moment, however, I'm a little confused about the output.

Say for example I'm fitting the following model:
y ~ m + cov1 + cov2 + E + SNP + SNP*E ------ (model 1)

What I'm really interested in is testing if the SNP*E is statistically significant. Normally, I would do a likelihood ratio test by subtracting the -2 log likelihoods of the full model (model 1) with a reduced model (model 2 below) to obtain a chi-square statistic with 1 degree of freedom.

y ~ m + cov1 + cov2 + E + SNP ------ (model 2)

I fit the full model (model 1). The output for ProbABEL provides a chi2_SNP statistic. I'm assuming that this is the chi-squared statistic from the LRT comparing the full model as I fit above with a null (no covariate) model (NOT to be confused with the reduced model)... is this correct? Verification of this would be helpful.

So if I wanted a p-value for the SNP*E interaction term comparing nested models (full - reduced) to get the LRT, how would I go about getting it with ProbABEL? (other than simply doing beta_SNP*E/se_SNP*E, which would be equivalent to doing a wald test)

Is it appropriate to fit model 1 then model 2 and then subtract the chi2_SNP values to get the SNP*E chi2 for 1 degree of freedom?

Thank you for your help.

Posts: 1
Joined: Fri May 29, 2015 10:35 pm

Re: Understanding probabel GxE interaction output

Postby wentingwu » Fri May 29, 2015 10:41 pm

I have the same question.
I hope to get an significant p value for G*E term. How to get that p value. And coefficients for that interaction term.

Posts: 2
Joined: Thu May 21, 2015 8:13 pm

Re: Understanding probabel GxE interaction output

Postby agiri_genepi » Wed Jun 17, 2015 8:46 pm


You can obtain the p-value for the interaction term by dividing the SNPxcovariate_interaction_beta by the SNPxcovariate_interaction_standard_error. This gives you a z-score which you can convert into p-value using R doing something like this for example: 2*pnorm(-abs(z-score). This should give you a two-sided p-value for interaction.

My original question however has not been answered at all, which is: What is the chi2_SNP test statistic for (in the event we run an interaction analysis):

a) the SNP
b) the interaction term of the SNP and the covariate
c) the whole model in question compared to the null-model.

Help with this query would be greatly appreciated.

Thank you much.

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Joined: Wed Jun 03, 2015 7:47 pm

Re: Understanding probabel GxE interaction output

Postby nfishbane » Mon Jul 20, 2015 8:28 pm

I have verified the ProbABEL results manually in R using the imputed dosages. It seems as though when running palinear with an interaction term (GxE), the chi2_SNP test statistic is a 2-df statistic from Likelihood Ratio Test. The full model is containing all 3 terms G, E, and GxE (plus the covariates), and the null model only contains E (with the same covariates). So it considers the "null model" to be without both genetic terms (G and GxE). I'll denote this as the Likelihood Ratio L(GxE)/L(E) which asymptotically approaches a 2-df chi-square statistic.

I think if we want P-values for the individual terms, we will need to perform either
1. A Wald Test using the coefficient and the SE
2. Another ProbABEL run without the GxE term to obtain a chi2_SNP statistic representing the 1-df L(G,E)/L(E) statistic. This way we can obtain, say, L(GxE) /L(G,E) chisq-statistic which is also 1-df, for a test of the interaction term by itself.

Hope this is helpful!

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Re: Understanding probabel GxE interaction output

Postby lckarssen » Wed Jul 22, 2015 12:30 pm

Thanks Nick! I hadn't had the time yet to check the code before answering.

I'll update the manual to make this more explicit.
Lennart Karssen
The Netherlands

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