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# Re: st: Analysing longitudinal data

 From K Jensen To statalist@hsphsun2.harvard.edu Subject Re: st: Analysing longitudinal data Date Tue, 26 Feb 2013 16:12:01 +0000

```Thanks Nick. As ever that is very helpful.
Kind regards
Karin

On 26 February 2013 15:39, Nick Cox <njcoxstata@gmail.com> wrote:
> 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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```