

Calculating Probabilities in growth m... 

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This is my first foray into Mplus and I am confused. I am estimating a logistic regression growth curve mixture model. I have estimated linear and quad growth parameters. I want to plot the curves (or lack of). I calculated the probabilities, as I would from estimates in logistic regression, For the first time point Y’ = 2.054 + (.02*Linear coefficient)+(.002*Quad coefficient) Y’ = 2.054 + (.02*1) + (.002*1) Then, Prob = EXP(Y’)/1 + EXP(Y’) Estimate Means I1 2.054 0.305 6.736 S1 0.020 0.072 0.282 Q1 0.002 0.004 0.481 The plots I get from doing this are different from the plots I get from when I plot the data from the section labeled, RESULTS IN PROBABILITY SCALE SAL1 Category 1 0.588 Category 2 0.412 Obviously I am confused as I had expected these would be the same. My questions are, 1. Which is correct! 2. What am I plotting (if anything) when I calculate what I thought were prob using just a logistic regression approach? 3. What am I plotting when I plot the data from the “Results in Probability Scale” section? 4. Finally a basic question  Am I predicting the 1 in data or Category 1 as indicated in Mplus? Thanks! 


If your growth model has growth factors (random effects) with nonzero variances the outcome probabilities have to be computed via numerical integration over the distribution of the random effects. This is what is done in the Probability output and in the graphics of Mplus. It is hard to do by hand. What you are computing is the outcome probabilities at the means of the growth factors. The two approaches are not the same with a nonlinear model such as this. Mplus creates categorical variable scales labeled 0, 1, ... Therefore category 1 in Mplus is 0 and category 2 is 1. With a binary outcome Mplus models the probability of category 2 as usual in logistic regression. 


Thanks for the speedy response! Perfect  I'm on track 

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