period to period change

[Buyadaa Oyunchimeg]

Dear Stata users,

The dataset I am working on is repeated measures data and it looks as below. I want to calculate period to period GFR change rather than change from "baseline".

[CODE]
* Example generated by -dataex-. To install: ssc install dataex
clear
input long ID float(Visit_months Years_from_baseline) double gfr int sbp
1   0         0  53.7 127
1   4  .3150685  43.8 117
1  12  .9561644  50.4 125
1  24 2.0027397  46.7 118
1  36  3.016438  38.1 115
1  48  3.978082  46.4 114
1  60  4.994521    37 102
1  72  5.991781    39 110
1  84  7.008219  34.1 110
1 100  7.282192  35.6 108
1 101 10.432877  30.5 119
1 103 13.238357  38.2 119
2   0         0 121.7 114
2   4  .3068493  87.2 114
2  12 1.0109589  74.6 112
3   0         0  79.6 146
3   4  .3150685  71.2 122
3  12 1.0109589  71.1 134
3  24 2.0219178  68.7 141
3  36  3.030137  46.5 144
3  48 4.0054793  43.5 120
4   0         0    54 159
4   4  .3260274  58.8 129
4  12  .9945205  58.7 113
4  24 2.0054793  49.6 119
4  36  3.005479  53.1 134
4  48  4.076712  53.8 117
4  60  5.008219    56 111
4  72   6.00274  57.3 118
4 100   6.19726  65.9 116
4 101  9.052054  56.3 116
end
[/CODE] 

I would really appreciate if anyone may help.

Thanks a lot.


Oyun

[Clyde Schechter]

by ID (Visit_months), sort: gen period_change_gfr = gfr - gfr[_n-1]
by ID (Visit_months): gen rate_change_gfr = period_change_gfr/(Visit_months-Visit_months[_n-1])

will give you both the absolute change in GFR (ml/min) over the period as well as the rate of change of GFR (ml/min per month).

[Buyadaa Oyunchimeg]

As always, thank you so much prof. Schechter.