How to calculate change from baseline in panel dataset

[Al Bothwell]

I have a panel dataset organized months: Month 0, Month 6, Month 12, and Month 18. Other variables in the dataset are participant ID, group, and sbp (the outcome variable). How do I calculate a new variable representing the change from baseline for the difference between Month 0 vs. Month 6, Month 0 vs Month 12, and Month 0 vs. Month 18? A sample dataset is attached. Your help will be appreciated.

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(id group time) double sbp
1 1  0 129.5
1 1  6 121.5
1 1 12   120
1 1 18   120
2 2  0   140
2 2  6   138
2 2 12   136
2 2 18   126
3 3  0 125.8
3 3  6 123.6
3 3 12 120.5
3 3 18   120
4 4  0 118.6
4 4  6 117.9
4 4 12 116.9
4 4 18 115.9
end

[Clyde Schechter]

assert inlist(time, 0, 6, 12, 18)
by id group (time), sort: gen byte change_from_baseline = sbp-sbp[1] if time[1] == 0

If an individual has no baseline observation, the change_from_baseline will be set to missing value for all observations of that individual.
Note: You have both id and group variables, and in the example data they are the same. I cannot tell whether it is the variable id or the variable group which identifies individual participants. I wrote the code with -by id group-, but if the identifier for an individual is just id, then it should be -by id-. And if the identifier for an individual is just group, then it should be -by group-.

[Al Bothwell]

Thank you!