I hope I can explain my problem well.
I have a big row of participants with 12 measuring points ( 1 for each month). The problem is that the baseline of these measurements, and so all the measuring points per participant are all in different months. I now need to count the measurements per month instead of per participant. The measurements go over a few years. I would like to get say the year 2009. Some participants started earlier and these measurements before 2009 should be put at the end of the row in the corresponding month. The same for participants that started later, these measurements after 2009 should be put in the corresponding months before they started.
Does anyone know how to do this without having to make a new dataset?