For my study I am doubting about the following:
First I want to present the incidence rate for patients who received an CT-scan.
I thought I would just count all the CT-scans that took place and divide this by the time all patients are at risk. So, e.g.:
Patient 1: 1 CT-scan in 365 days; Patient 2: 3 CT-scans in 250 days; Patient 3: 1 CT-scan in 10 days; Total --> 5/625 = 0.008
But then I thought about what would happen if I calculated the incidence rate for each patient separately and add this (instead of just the total as showed above).
We would get something like this for the patients above:
1/365 + 3/250 + 1/10 = 0.002739 + 0.0120 + 0.1 = 0.114739
What I understand is that the first way gives more weight to patients who are a long time at risk for getting a CT-scan, while the second way gives more weight to patients who are a short time at risk for getting a CT-scan.
What I don't know is if it is even justified to use the second way of calculating? And where I can't get my head around is what it means for my results if I use the second way instead of the first way - what is the difference between the first answer (0.008) and the second answer (0.114739)?