Hi statisticians,

I am new to longitudinal data analysis, and currently learning by myself to be able to carry out hypothesis testing.

I have a short longitudinal data in terms of limited repeated measures (only baseline and time-1 post baseline are available). Also there is a group indicator ("studygroup") to suggest control versus intervention group. The goal of analysis is to evaluate if the trend of outcome (from baseline to time-1) differs between groups (control vs. intervention).

Variables:
  • outcome: meanscore_sds (calculated based on a scale, ranging from 1-5)
  • time variable: time (0: baseline; 1: time-1)
  • group indicator: studygroup (0: control; 1: intervention)
  • sex: (0: boy; 1: girl)
  • id variable: surid
Code:
global restrict = "t1dropcase == 0 & t2dropcase == 0" // limit to eligible analytical sample 
mixed meanscore_sds time##studygroup if $restrict || surid: , residual(uns, t(time)) var ml
Results from above model:
Code:
                                                Wald chi2(3)      =      17.65
Log likelihood = -6160.3764                     Prob > chi2       =     0.0005

---------------------------------------------------------------------------------
  meanscore_sds |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
----------------+----------------------------------------------------------------
         2.time |   .0660626   .0308159     2.14   0.032     .0056645    .1264606
                |
     studygroup |
  Intervention  |   .0048707     .03559     0.14   0.891    -.0648845    .0746259
                |
time#studygroup |
2#Intervention  |   .0393069   .0432313     0.91   0.363     -.045425    .1240387
                |
          _cons |   4.276469   .0253691   168.57   0.000     4.226747    4.326191
---------------------------------------------------------------------------------

------------------------------------------------------------------------------
  Random-effects Parameters  |   Estimate   Std. Err.     [95% Conf. Interval]
-----------------------------+------------------------------------------------
surid: Identity              |
                  var(_cons) |   .0984436   1.990941      6.00e-19    1.61e+16
-----------------------------+------------------------------------------------
Residual: Unstructured       |
                     var(e1) |    .682875   1.991058      .0022516    207.1039
                     var(e2) |   .5772798   1.991008      .0006693     497.893
                  cov(e1,e2) |   .0536583   1.990978     -3.848587    3.955904
------------------------------------------------------------------------------
LR test vs. linear model: chi2(3) = 123.58                Prob > chi2 = 0.0000
My interpretations:
  • coefficient for "time": among control group, outcome change at time1 compared to baseline
  • coefficient for "studygroup": outcome in intervention compared to control control at baseline
  • but I am not sure how to interpret the coefficient for interaction term. Is it the difference of baseline-time1 trend between two groups? Or is it the difference of outcome at time1 between intervention and control?
Thank you so much!!

Best,
Mengmeng