I have prepared a linear growth model to estimate genetic and environmental influences (ACE) in the intake and the slope with ordinal twin data: 3 variables, 7 categories in each of them. The sample is is divided in 6 groups (and so is the script), according to zygosity and sex: MZ men, DZ men, MZ women, DZ women, DZ man-woman and DZ woman-man.
I have been reading from: Mehta PD, Neale MC, Flay BR. Squeezing interval change from ordinal panel
data: latent growth curves with ordinal outcomes. Psychol Methods. 2004 Sep;9(3):301-33; and I understood that the thresholds must be held invariant in order to specify the mean structure. Is this appreciation correct?
I also have tried to adapt a script to do this latent growth model calculations, but the sections concerning the definition of the thresholds doesn't seems to work and the execution of the script returns:
" Error: The algebra 'ACE.ThInc' in model 'lgcOrdACE' generated the error message: non-conformable arguments"
Any help to interpret the meaning of these "invariant thresholds" as well as this error message would be very much appreciated. Is there any possibility to solve it? I have attached the script with the model to help with the issue.
Thanks a lot in advance!