Hi May,
I do not have Stata to test the following solution it may need some tweaking:
sort id age
by id: gen total_duration =age[_N] - age[1]
by id: gen average_freq=total_duration /_N
hope this helps,
salah mahmud
On Tue, Jun 10, 2008 at 12:45 PM, MAY BAYDOUN <[email protected]> wrote:
> 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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