Dear Martin,
Thanks a lot for your help. I think I got it. The code
is:
sort id age_visit
drop total_duration average_freq
by id: gen total_duration =age_visit[_N] -
age_visit[1]
by id: gen average_freq=1/(total_duration /_N)
The frequency represents the average number of times
per year that a person is examined. So, if it is once
a year, it would be one, if it is once every two
years, it would be 0.5 etc.
By applying this code I obtained a mean duration of 22
years and a mean frequency of 0.6 which is about
right. Thanks again!
May
--- Martin Weiss <[email protected]>
wrote:
> Duration is the easy one here:
>
> bys id: g duration=ageatvisit[_N]-ageatvisit[1]
>
> But I just do not know what your frequency is
> supposed to mean. Please
> specify...
>
> Martin Weiss
>
_________________________________________________________________
>
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> 72074 Tuebingen
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>
> Home:
>
http://www.wiwi.uni-tuebingen.de/cms/index.php?id=1130
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> Publications:
>
http://www.wiwi.uni-tuebingen.de/cms/index.php?id=1131
>
> SSRN:
>
http://papers.ssrn.com/sol3/cf_dev/AbsByAuth.cfm?per_id=669945
>
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On
> Behalf Of MAY BAYDOUN
> Sent: Tuesday, June 10, 2008 7:46 PM
> To: [email protected]
> Subject: st: Duration and average frequency of
> examination
>
> Dear statalisters,
>
> I was wondering if you could help me figure out a
> way
> to obtain the frequency at which each individual was
> examined (i.e. average number of follow-up visits
> per
> year) and the total duration (i.e. time between
> first
> and last visit) if the dataset is person-period
> format
> and I have the following structure:
>
> id visit age at visit
> 1 1 30
> 1 2 32
> 1 3 36
> 1 4 50
> 2 1 45
> 2 2 46
> 3 3 50
> 3 4 56
> 3 5 60
> 3 6 65
> 4 1 42
> 4 2 48
>
> In other words, the distance in age between visits
> varies for each individual, the number of visits is
> not the same and the age at baseline varies as well.
>
>
> So, I would like to obtain the following variables
> (made up numbers for frequencies):
>
> id Freq Duration
> 1 0.5 20
> 2 0.4 1
> 3 0.3 15
> 4 0.6 6
>
> The duration for individual 1 for example is:
> 50-30
>
> The average frequency is the tricky one I guess, it
> is
> for individual 1:
> [1/(32-30) + 1/(36-32) + 1/(50-36)]/4
>
> Thanks a lot and hope to hear from you soon,
>
> Sincerely yours,
>
> May
>
>
> May Baydoun, PhD in Epidemiology (UNC-Chapel Hill)
> Staff Scientist,
> National Institute on Aging, NIH/IRP, Biomedical
> Research Center,
> Baltimore, MD
>
>
>
>
> May Baydoun, PhD in Epidemiology (UNC-Chapel Hill)
> Staff Scientist,
> National Institute on Aging, NIH/IRP, Biomedical
> Research Center,
> Baltimore, MD
>
>
>
> *
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> *
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>
> *
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>
May Baydoun, PhD in Epidemiology (UNC-Chapel Hill) Staff Scientist, National Institute on Aging, NIH/IRP, Biomedical Research Center, Baltimore, MD
*
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