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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: Response curves of the first four individuals

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?

[edit] included a column of timepoints

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  • $\begingroup$ What is the x-axis in this plot? Is there a "time" associated with each of the repeated measurements? $\endgroup$ – Macro Jul 11 '13 at 15:33
  • $\begingroup$ The x-axis denote time. Measurements were taken each week. $\endgroup$ – Stijn Jul 11 '13 at 16:09
  • $\begingroup$ The layout of your data set at the top of the post does not include the time variable. Do you have this information? $\endgroup$ – Macro Jul 11 '13 at 17:09
  • $\begingroup$ 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?) $\endgroup$ – Stijn Jul 11 '13 at 19:03
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    $\begingroup$ 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? $\endgroup$ – chl Jul 11 '13 at 19:55

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