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st: How to count occurrences of specific value

From   "Jia Peng" <>
To   <>
Subject   st: How to count occurrences of specific value
Date   Wed, 1 May 2013 20:03:54 +0800

Dear All,

I have a data set with the following structure,

id		date			flag
95001	14jun2000	1
95001	12apr2000	1
95001	16mar2000	0
95001	16nov1999	0
95001	10may1999	1
95001	30mar1995	0
95002	01nov1989	0
95002	01mar1985	1
95002	01jun1983	0
95002	01may1983	1
95002	01dec1982	0
95002	01oct1982	0

And now, I would like to generate a new variable, say temp, which represents
for each observation how many times flag == 1 has occurred within the same
id from five years ago to the date specified, i.e., for the first
observation, I want to count how many times flag == 1 has occurred with the
id 95001 between 14jun1995 and 14jun2000.

I have tried to loop over every observation using the following code,

gen temp = .
local N = _N
forvalues i = 1(1)`N' {
count if flag == 1 & id = id[`i'] & (date[`i'] - date)/365.25 <= 5 &
(date[`i'] - date)/365.25 >= 0
replace temp = r(N) in `i'

However, there are half a million observations in the entire data and the
above code cost hours of time. Is there any way to solve the above problem
more efficiently?

I have also tried to use -egen-, but all I can get is how many times flag ==
1 has occurred with the same id. Is there any way to take into consideration
different date ranges in this context?

Any thoughts?

Peng Jia

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