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# Re: st: How to count occurrences of specific value

 From Michael Barker To statalist@hsphsun2.harvard.edu Subject Re: st: How to count occurrences of specific value Date Thu, 2 May 2013 11:21:57 -0400

```It looks like you are comparing each observation to every other
observation in your data set. If your data are sorted, you only have
to look back within each 5-year window for each id. It looks like your
data are sorted descending by date, so the code would look like this:

gen temp = flag
local N = _N
forvalues i = 1(1)`N' {
local j=`i'+1
while (id[`i']==id[`j'] & (date[`i'] - date[`j'])/365.25 <= 5) {
replace temp = temp[`i'] + temp[`j']
local j = `j'+1
}

If your data were sorted ascending by date, you would just iterate j
downwards (j = i-1, j=j-1) and start the "forvalues" loop at 2
(forvalues 2(1)`N' {)

Mike

On Wed, May 1, 2013 at 8:31 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> Your code wouldn't work as
>
> id = id[`i']
>
> should be
>
> id == id[`i']
>
> but I presume you just copied it incorrectly.
>
> I don't know a sure-fire way to speed this up. It might just be faster
> if you -expand-ed the data to one observation for every day. Then the
> code would be  simpler,  but you would end up with several millions of
> observations. Or you could translate the code into Mata.
>
> I don't think -egen- will help you here at all. The essence of the
> problem is comparing each observation with others for the same
> identifier, and -egen- stops at simple recipes of that kind.
> Nick
> njcoxstata@gmail.com
>
>
> On 1 May 2013 13:03, Jia Peng <jiapengcass@gmail.com> wrote:
>> 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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```