Asta B. posted on Tuesday, August 02, 2011 - 6:07 am
I just started working with Mplus (great program, actually) and want to compare results of cluster analysis in SPSS with more or less restrictive LPA.
I got no problem performing the exploratory kind but the confirmatory variations are difficult for me as a novice using Mplus. I managed to "reproduce" the response patterns but now I'd like to fix class proportions - just to see how that works.
Please correct me if I did it the wrong way.
I translated the 5 class proportions I found in the CA (42.1%, 11.4%, 2.2%, 1.5%, 42.9%) into thresholds (-0.319, -2.050, -3.795, -4.185) by using theta = -ln((1/p)-1). In the output I found my thresholds again as "means of categorical latent variables" but under the heading "final class counts and proportions for the latent classes based on the estimated model" (38.4%, 6.8%, 1.2%, 0.8%, 52.8%) and "...estimated posterior probabilities" (38.2%, 7.5%, 0.2%, 1.7%, 52.5%) the values are not like my original proportions. Thought they should be. What’s wrong: my analysis or my interpretation?
I think I read somewhere at this board about a similar problem but I couldn't transfer the answer to my case, so I would appreciate your help.
Btw: I am talking about alcohol use patterns and my cluster 5 (42.9%) are the abstainers.
If you have the class proportions, the correct formula is:
logit = log (pclassj/pclassJ)
where classJ is the last class.
Asta B. posted on Wednesday, August 03, 2011 - 2:03 am
Ah, okay, thank you! I tried that and the analyses ran well.
But I still got some difficulties interpreting the results. Would you mind explain that to me also?
The order of magnitude of the final class proportions is like the one in my CA, but I don't know why the values are different. If that is a consequence of estimation why should I fix the values in the first place?