Assigning occasions to sparse data

Hi everyone; I'd like to get some feed-back from the group about how to assign occasions for the estimation of inter-occasional variability. The situation is this: I have a large Phase 1 dataset (both single dose and multiple dose) where many samples were taken per subject, and "occasion" is readily assigned by using period. The maximum number of occasions in these data is equal to 4. However, I also have a dataset from a Phase 3 study where sparse samples were taken. As you would expect, the amount of data from patient to patient is quite variable, with some subjects having as few as 2 samples where others have as many as 8. Also, the times (relative to dosing) are quite variable. What I would like to do is to combine these two datasets, and keep the same number of occasions that are in the rich dataset by grouping these sparse samples by time relative to the first dose. For example (and this is arbitrary) I could define Occasion 1 as including any sample taken between 0-48 hours, Occasion 2 as including any sample between 48-72 hrs, etc. up to Occasion 4. Does anyone see a problem with this? Or, do you have a better idea? Many thanks for your time, Mike Fossler GSK