I am trying to implement a mixed-effects regression model in Matlab to see the correlation between self-reported stress levels and some physiological features. Data comes from a longitudinal study so I have to deal with repeated measures' issues.
I have some knowledge about basic regression models such as OLS or PLSR but I am new to mixed-effects regression models, and I am completely lost. I don't find any good example and I don't know where to start from.
I currently have a data matrix of 34 people with 30 measures of 65 variables for each one of them, and I have several questions.
- Do I have to select a single/or a few variables to regress or can I try to include all the variables in the model?
- Do I have to think about a model (formula) or can Matlab automatically create one?
- Which is(are) the random effect variable? The subjects' id-s?
- How do I know which interaction term to introduce?
- How do I know if I should use a random intercept or s random variable?
I would be very grateful if you could give me some steps to follow.