I am pretty sure the two are connected. By setting -1 as the reference, the matrix is set up so that the last entry (set to zero) is the reference. This disturbs the ordinal nature of your data, as the estimate of the correlation used in AR(1) is dependent on the ordering and not on an index. As a result, rho is estimated from values at -4, -3, -2, 0, 1, 2, 3, -1, rather than the ordinal values from -4 to 3. That will increase the standard error in this case, and cause a small change in the point estimates.
If you wish to compare the other timepoints to -1, there are better ways than setting that group as a reference and examining the solution vector, and those preserve the ordinal nature of your data. You can compare the LSmeans at the various other timepoints to that timepoint by using an LSMESTIMATE statement, like this:
proc mixed NOCLPRINT NOITPRINT;
class user_ID Avg_Num; /* (ref='-1'); -> removed as a midrange reference point can create problems when you start creating LSMEANS */
model outcome = Avg_Num / SOLUTION;
repeated Avg_Num/ subject=user_ID type=ar(1); /* not critical, but it makes it apparent immediately what the repeated factor is */
lsmeans avg_num/cl;
lsmestimate avg_num '-4 v -1' -1 0 0 1 0 0 0 0,
'-3 v -1' 0 -1 0 1 0 0 0 0, '-2 v -1' 0 0 -1 1 0 0 0 0, '0 v -1' 0 0 0 1 -1 0 0 0, '1 v -1' 0 0 0 1 0 -1 0 0, '2 v -1' 0 0 0 1 0 0 -1 0, '3 v -1' 0 0 0 1 0 0 0 -1/ cl /* add in any multiple comparisons method if you want */;
run;
SteveDenham
... View more
