Quick question re. multiple imputation. I have a database with 1800 participants for one year of a longitudinal study. 1400 of these responded to a self-admin questionnaire. So I have 400 non-responders about whom I have basic demographic info (because they responded to previous years of the longitudinal study) and also some partial respoonders who answered the questionnaire but missed some items.
I've been advised to do multiple imputation. Should I do it for a) the whole sample 1800, or b) only those who answered the questionnaire?
My main outcome variable has about 30% missing values if I take the sample as a whole and 8-10% missing values is I take only responders to the questionnaire.