GEE with gamma family and log link
Except for the details of conditional vs marginal effects (discussed in a previous thread), the interpretation of fixed effects of a model with a log link (GLM(M), GEE, etc.) are not really a mixed-model-specific issue, but apply to any model with a log link. Your interpretations below sound correct (although you say something about "multiplying the arithmetic mean" by exp(beta); this is true, but it's also true that you would equivalently be multiplying any sensible location parameter (such as the geometric mean) by the same factor
On 11/12/21 8:32 PM, Tahsin Ferdous wrote:
I am fitting a generalized estimating equation with gamma family and log link. I am using GEE (geeglm function) from the R pcakage ?geepack? with gamma family and log link and unstructured correlation structure. Here, response variable or outcome is IFN_gamma_protein_pg_mg . Exposure is intervention or probiotic use. Another covariate is Timepoint. My code and the output is as mentioned below. m8<geeglm(IFN_gamma_protein_pg_mg~Intervention+Timepoint,data=B,family=Gamma(link=log),id= Participant_ID,corstr="exchangeable") summary(m8) * IFN_gamma_protein_pg_mg* *Predictors* *Estimates* *p* (Intercept) 0.01 *<0.001* Intervention [Probiotics] 0.34 *0.029* Timepoint [T2] 0.99 0.979 Timepoint [T3] 5.30 0.059 Timepoint [T4] 0.48 *0.039* Timepoint [T5] 0.11 *<0.001*
I am trying to interpret the coefficients as follows: For every one-unit increase in the probiotic across the population, the log average of IFN_gamma_protein increases by 0.34 units. The exponentiated coefficient ( exp )= (exp(0.34)=1.41) is the factor by which the arithmetic mean outcome on the original scale multiplied, i.e., when intervention is probiotic, for every one-unit increase in the probiotic across the population, the average of IFN_gamma_protein on the original scale is 1.41 times higher compared to when intervention is control within levels of other variable. Similarly, for timepoint 2, the average of IFN_gamma_protein on the original scale is exp( )= exp(0.99)= 2.69 times higher compared to timepoint 1 within levels of other variable. For time point 3, the average of IFN_gamma_protein on the original scale is exp( )= exp(5.30)= 200.34 times higher compared to timepoint 1 within levels of other variable. Can someone confirm me that I am in the right track in the interpretation of parameters? I am posting here as I also want to fit a Gamma GLMM. [[alternative HTML version deleted]]
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