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Mplus Discussion > Growth Modeling of Longitudinal Data >
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 Gareth posted on Monday, January 19, 2009 - 8:35 am
In growth mixture modeling, could the KNOWNCLASS option be used to specify a class of people who scored zero on the outcome for every time point?
 Linda K. Muthen posted on Monday, January 19, 2009 - 10:38 am
No. Instead set the thresholds in one class to +15. In that class, also fix the means and variances of the growth factors to zero.
 Gareth posted on Wednesday, January 21, 2009 - 2:38 am
The outcome variable is continuous, not categorical. Should I therefore fix the mean to 0 and the variance to 0 for y1-y3, in class 1?
 Gareth posted on Wednesday, January 21, 2009 - 2:42 am
The outcome variable is continuous, not categorical. Should I therefore fix the mean to 0 and the variance to 0 for y1-y3, in class 1?

%c#1%
[y1-y3@0];
[i@0 s@0];
s@0 i@0;
 Linda K. Muthen posted on Wednesday, January 21, 2009 - 10:02 am
This approach does not work with a continuous outcome. You might instead consider two-part modeling. See Example 6.16 in the Mplus User's Guide.
 Gareth posted on Saturday, January 24, 2009 - 5:33 am
I have modified example 6.16, but is the floor variable treated as a latent class in its own right (so I should ask for classes=5), or is it present throughout, in the overall model (so I should ask for classes=4).

I anticipate four latent classes (increasing, decreasing, rapidly increasing, consistently low), in addition to the "floor" variable (no use)?

Ideally, I want to compare the four growth patterns to "no use". If this is not possible, I can use the consistently low class instead, but I would like to know how the zeros are accounted for in two-part mixture growth models.

CLASSES = c (4)!
MODEL: %overall%
iu su | bin1@0 bin2@1 bin3@2;
iy sy | cont1@0 cont2@1 cont3@2;
iy sy ON x;
su@0; iu WITH sy@0;
 Bengt O. Muthen posted on Saturday, January 24, 2009 - 4:06 pm
Two-part modeling does not involve a class per se. So if you hypothesize 4 classes, then classes = c(4);

Only if you also hypothesize a class of zero (floor value) throughout would you specify a zero class for the u part; but that you have to add yourself.
 Gareth posted on Monday, January 26, 2009 - 9:39 am
I hypothesize a class of zero throughout, so I have attempted to add a zero class for c#1. I am expected the other four classes to show different growth patterns, but a zero value for the first class throughout. Is this the correct syntax?

CLASSES = c (5);
MODEL: %overall%
iy sy | cont1@0 cont2@1 cont3@2;
iu su | bin1@0 bin2@1 bin3@2;
iy sy ON x;
%c#1%
iy sy | cont1@0 cont2@1 cont3@2;
iu su | bin1@0 bin2@1 bin3@2;
su@0; iu WITH sy@0;
 Bengt O. Muthen posted on Monday, January 26, 2009 - 5:01 pm
No, you should say

%c#1%
iu-su@0;
[iu-su@0];
[bin1$1-bin3$1@15];

- check in the Mplus plot that this gives you a zero class.
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