# Re: st: Duration and average frequency of examination

 From "Salah Mahmud" To statalist@hsphsun2.harvard.edu Subject Re: st: Duration and average frequency of examination Date Tue, 10 Jun 2008 16:51:36 -0500

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 <mbaydoun2002@yahoo.com> 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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