I aimed at studying the effects of an exposure (E) on fetal growth estimated by repeated ultrasound measurements (n = 2 measures per participant in the following example).
I used interaction terms of gestational age with exposure, in order to compute the effect of exposure at different age (using
lincom in Stata).
I am surprised because the standard error of the effect estimates computed with lincomest (performed after xtmixed) are similar at 22.5 and 32.6 gestational weeks (both SEs = 8, while Beta = - 1 and - 19 g at 22.5 and 32.6 gestational weeks, respectively). I am wondering if I correctly used lincomest.
gen expoAG = expo*AG /* Gestational Age */ gen expoAGsq = expo*AGsq /* squared of Gestational Age */ xi : xtmixed EFW expo AG AGsq expoAG expoAGsq if go_pen==1 ||n_ident:, mle lincomest _b[expo] + _b[expoAG]*(22.5) + _b[expoAGsq]*(22.5*22.5) /* Prediction at 22.5 GW */ xi : xtmixed EFW expo AG AGsq expoAG expoAGsq if go_pen==1 ||n_ident:, mle lincomest _b[expo] + _b[expoAG]*(32.6) + _b[expoAGsq]*(32.6*32.6) /* Prediction at 32.6 GW */