In an ITT randomized trial, there are one baseline response (y0) and 5 post-treatment follow-ups (y1...y5). Although the number of post-treatment follow-ups is predetermined, participants can drop out of the study. So, for example, ID 1 has responses y0, y1, y2. ID 2 has responses y0, y1, y2, y3, y4,y5. ID 3 only has response y0. I am planning to use linear mixed model for analysis.
In that case, do I need to impute the missing responses for each participant e.g. via last observation carried forward? Or no imputation needed for people who had at least one post-treatment response, which already satisfies the ITT requirement?
What if the missing response is at baseline, is multiple imputation the most common way?