

Longitudinal measurement invariance 

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Hi Bengt and Linda, I'd like to conduct a test of measurement invariance for daily diary data based on mood assessments. I have roughly 30 time points (although some individuals have missing data at various time points), and 200300 participants. I’ve seen discussion of longitudinal measurement invariance here and elsewhere. But those examples typically focus on a much smaller number of time points. I'm wondering if the following is the most appropriate approach? *Treat each time point as a factor, allowing the factors to correlate. *Test configural invariance first, before evaluating the change in model fit once factor loadings etc. are constrained to be equal across time points. My concern is that as a single level analysis, I will have a lot of factors to correlate  will the model encounter problems if my sample is 200300 participants? Is there another way to do this analysis (e.g., MLM with time points at level 2 and a clustering effect for individuals as well?) Kind regards, Matthew 


How many variables do you consider per time point and how many factors? 


Hi Bengt, There are six items for the scale, and two factors provide a good fit for this measure. I wasn't intending to use other variables in addition to this mood measure. 


So you have 6 x 30 = 180 variables if you do it as a wide analysis with a longitudinal factor model. That will be heavy and won't work well with your smallish sample. You can do it as a twolevel model, so 6 variables, 30 "cluster members", and subject as the level 2 unit, that is, using cluster=id. But then you don't get a test of measurement invariance across time as you would in a wide analysis. So maybe you can take a wide approach and choose a few critical time points such as beginning, middle, and end in order to have fewer variables. Testing the longitudinal invariance that way. 


Terrific. Thank you for your help Bengt. 

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