I have a group of physicians that want to test whether or not the education of staff and making access to a particular medication/device more available to a certain population is statistically different than the previous standard of care. They are measuring 3 months of data at baseline, then implementing education and access changes and then measuring 3 months of more data. The intervention takes a couple of months to educate and get the access up and running. During the first month they are looking at patients that had a particular event in the previous 26 months and whether or not they had this particular medication/device in place, during the second month the same collection takes place, and then during the third month the same data collection takes place. This is all taking place using Electronic health records. From my understanding one patient could be counted in all 3 months if they met the criteria and patients could fall off during the second month if they are outside the window and be replaced with the same number of new patients that meet the criteria. This would result in the numbers potentially looking the same even though it is different patients.
The data collection after intervention would be the same process. You could also have patients in the post intervention data collection that were in the pre as well if their event was recent. I am unsure whether to look at it on a month by month basis or look at it from a 90 day window and count all those patients then look at a 90 day window after the intervention. Thanks for any input.