# Re: st: Duration and average frequency of examination

 From MAY BAYDOUN To statalist@hsphsun2.harvard.edu Subject Re: st: Duration and average frequency of examination Date Wed, 11 Jun 2008 10:37:38 -0700 (PDT)

```Dear Salah,

Thank you so much for your suggestion. I will try it
out.

Best,

May
--- Salah Mahmud <salah.mahmud@gmail.com> wrote:

> 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
> >
> >
> >
> > *
> > *   For searches and help try:
> > *
> http://www.stata.com/support/faqs/res/findit.html
> > *   http://www.stata.com/support/statalist/faq
> > *   http://www.ats.ucla.edu/stat/stata/
> >
> *
> *   For searches and help try:
> *
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> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

May Baydoun, PhD in Epidemiology (UNC-Chapel Hill)  Staff Scientist,  National Institute on Aging, NIH/IRP,  Biomedical Research Center,  Baltimore, MD

*
*   For searches and help try:
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*   http://www.ats.ucla.edu/stat/stata/
```