st: summarize by different levels/groups with -egen- ?

[Patricia Biedermann <pati.stat@gmail.com>]

Dear STATA users,

I want to summarize following:

School		Class		Pathogen		
A			A1			H
A			A1			T
A			A1			H
A			A2			S
A			A2			H
A			A3			K
A			A3			I
B			B1			S
B			B1			T
B			B2			H
	
I’ve visited different classes in different schools. In each class I
checked if the children were infected with some kind of pathogen.
-	I found e.g that in class A1 two children were infected with pathogen H.
-	Now, I want to summarize that I just found pathogen H in class A1
WITHOUT the actual amount of pathogen itself (2 times in this case);
Basically “Was pathogen H found in class A1” = yes or no; Finally, the
information should be presented at school level. ("How many classes in
school A pathogen H was found?)

So far I tried egen, bysort / =_n==N and commands. I also created
dummy variables for each pathogen.  It never worked out the right way.
Maybe it’s just an error in reasoning.

THANKS A LOT in advance.

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