Hi,

I am working on a difference in difference analysis based on a panel data of 5 rounds. The treatment, say A, was introduced between rounds 2 and 3. Thus rounds 1,2 are pre treatment rounds, and rounds 3,4,5 are post treatment rounds. The difference in difference estimator would be obtained from an interaction between dummy for post treatment rounds,treatment intensity, months of exposure to treatment.

Now given this framework, months of exposure is negative for pre-treatment rounds. I am confused whether to code these negative values as 0 or missing.

Also, should I code treatment intensity in pre-treatment rounds as zero or missing? I have currently coded them as missing.

Any help in this regard would be greatly appreciated.

Thanks,