# st: Duration and average frequency of examination

 From MAY BAYDOUN To statalist@hsphsun2.harvard.edu Subject st: Duration and average frequency of examination Date Tue, 10 Jun 2008 10:45:56 -0700 (PDT)

```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

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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```