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# Re: st: Analysing longitudinal data

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: Analysing longitudinal data Date Tue, 26 Feb 2013 15:39:33 +0000

```I don't think any programming is needed here at all, just a few
commands issued in a do-file or interactively.

Your first two questions are of the form

{count|average} in moving time interval

and both are addressed in moderate detail within

SJ-7-3  pr0033  . . . . . . . . . . . . . .  Stata tip 51: Events in intervals
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
Q3/07   SJ 7(3):440--443                                 (no commands)
tip for counting or summarizing irregularly spaced
events in intervals

http://www.stata-journal.com/sjpdf.html?articlenum=pr0033

The last has been discussed in various places, but rather than
references here is some sample code.

gen NotWanted = (value < 1) | missing(value)
bysort  ID (NotWanted Date) : gen TimeSinceLast = Date - Date[_n-1] if
!NotWanted

The logic looks a little contrived, but the idea is to get a sort
order that looks like this, so that the time since the
last previous event is just the current date minus the previous date.
For individual, there will be only one such date,
30/04/2011, for which the answer will be 2 days. The answer for the
first will be missing.

1       1.5     28/04/2011
1       1.3     30/04/2011
1       0.6     02/03/2010
1       0.6     24/05/2010
1       0.6     16/08/2010
1       0.6     10/11/2010
1       0.9     09/02/2011
1       0.7     01/05/2011
1       0.6     03/05/2011
1       0.6     09/05/2011

Once you have values in a variable, -summarize- gives you the mean.

Nick

On Tue, Feb 26, 2013 at 3:18 PM, K Jensen <k.x.jensen@gmail.com> wrote:

> I have a dataset that looks like the sample I have pasted in at the
> end of this message, with repeated measures over time on the same
> individuals.
>
> I would like to answer questions like:
> When a value was over 1, how many were measured again within the next
> week? How many of those were also over 1?
> What was the average interval between a value being over 1 and the
> next value for that individual?
>
> How would I go about programming this in Stata?
>
> Thankyou
>
> Karin
>
> ID      Value   Date
> 1       0.6     02/03/2010
> 1       0.6     24/05/2010
> 1       0.6     16/08/2010
> 1       0.6     10/11/2010
> 1       0.9     09/02/2011
> 1       0.7     01/05/2011
> 1       1.5     28/04/2011
> 1       1.3     30/04/2011
> 1       0.6     03/05/2011
> 1       0.6     09/05/2011
> 2       0.6     14/04/2009
> 2       0.7     23/07/2009
> 2       0.6     16/11/2009
> 2       0.6     29/12/2009
> 2       0.6     30/03/2010
> 2       0.9     01/07/2010
> 2       1.2     27/09/2010
> 2       0.7     12/10/2010
> 2       0.7     26/10/2010
> 2       0.8     25/03/2011
> 3       0.3     21/01/2009
> 3       0.3     05/11/2009
> 3       0.1     01/03/2010
> 3       0.2     24/05/2010
> 3       0.2     18/08/2010
> 4       0.9     16/02/2010
> 4       1.0     01/03/2010
> 4       0.9     12/03/2010
> 4       1.1     25/03/2010
> 4       0.5     01/06/2010
> 4       0.4     31/08/2010
> 4       0.6     30/11/2010
> 4       0.6     19/04/2011
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