# Growth mixture modelling with unequal number of measurements per individual

I have responses to a questionnaire item from a number of people, measured at equidistant timepoints. I wish to fit a growth mixture model (in R, using the LCMM package) to this data to find latent classes. My data looks something like this:

    ID  item-response  timepoint
-----------------------
1       3           1
1       2           2
1       2           3
2       2           1
2       3           2
2       2           3
2       1           4
2       1           5
2       3           6
2       2           7
2       2           8
2       2           9
2       1           10
2       4           11
2       2           12
3       1           1
3       1           2
3       1           3
3       1           4
3       1           5
.       .           .
.       .           .
.       .           .


The item is one of 13 on a questionnaire on mood states. Responses are given on a Likert-scale (1 to 5).

A plot of the response curves of the first four individuals looks like this:

I am worried about the fact that the number of measurements per person is not the same. Is this a huge problem for growth mixture models or not so much?

 included a column of timepoints

• What is the x-axis in this plot? Is there a "time" associated with each of the repeated measurements? – Macro Jul 11 '13 at 15:33
• The x-axis denote time. Measurements were taken each week. – Stijn Jul 11 '13 at 16:09
• The layout of your data set at the top of the post does not include the time variable. Do you have this information? – Macro Jul 11 '13 at 17:09
• I've added a column of timepoints to make the data more clear. As you can see, the person with ID 1 only has three measurements, while ID 2 has twelve. So at some point, certain individuals stopped, while others kept providing measurements. (Question: do we call data like this censored data?) – Stijn Jul 11 '13 at 19:03
• Are these data responses to a Likert-type item? If so, could you please tell us a little more on the questionnaire that's being used? – chl Jul 11 '13 at 19:55