I have a latent growth curve model and wanted to see if the intercept and slope means differed between two groups, so I used the MODEL TEST command. When I run a test simultaneously looking at the two latent constructs, the Wald Test is significant:
MODEL TEST: 0 = i1 - i2; 0 = s1 - s2;
However, when I run a MODEL TEST for each comparison separately, neither one is significant. I am not sure how to interpret this. Any advice?
While I understand that interpretation, it does not really help me understand the pattern of differences I have found. Neither intercept nor slope differ between the two groups when tested independently BUT when tested together, there is a group difference identified (both by MODEL TEST and by looking at the chi-square difference test of nested models). Can I say that group 1 and group 2 differ in their model of growth? (The intercept and slope are significant. If the significant difference identified when testing the two latent means together, then group 1 shows a lower baseline value and a shallower growth over time than group 2).
Testing both together tests whether the line is equal across groups and this is a stronger hypothesis than testing either intercept or slope. The joint test would be the same as the two separate ones only if the two tests were uncorrelated.