Dear OHDSI Community,

My name is Saptha from Gilead Sciences, I am an Associate Data Scientist working on the Real World Evidence team.

I am working on the incidence rate analysis using ATLAS, where I have one target cohort and a few outcome cohorts. I am interested in calculating the overall incidence proportion of an outcome as well as by time period post index (0-3, 0-6, 0-9, 0-12, 0-24 months).

The overall incidence proportion is calculated with the following settings

```
- Time at risk starts with start date plus 0 days
- Time at risk ends with end date plus 0 days
- Study window begins on 2017-01-01 and ends on 2022-12-31
```

The incidence proportion for time period post index is calculated with the following settings. lets say for example 0-3 months

```
- Time at risk starts with start date plus 0 days
- Time at risk ends with start date plus 90 days
- No Study window is defined
```

I am noticing the overall incidence proportion is lower when compared to the time period post index. Can you please advise why the numbers are lower and also suggest how to define the IR analysis to have the same denominator for both overall as well as by time period post index.

Thanks a lot for all your help in advance.

Regards,

Saptha