st: Count the number of changing string values

[Ching Wong <ching.y.wong@student.adelaide.edu.au>]

Hi,

I have the following data set:

Id     sex   drug1999   drug2000    drug2001      drug 2001

1       F       Yes            No          Yes            Unknown

3       M       No           Yes          Yes             No

5      F         Yes          Yes          Yes            Yes

9      M       Unknown    Yes          No             Yes

10     F        Yes           No           Yes             No

I would like to produce a count of to show the number of people who
quit each year, only count the first time they quit and do not care if
they change their status to ‘No’ again.

What would be the command? I tried to change it to the long formal and
use the lag function, but it doesn’t work.

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