Re: st: missing events in stset

[Sara Mottram <s.mottram@cphc.keele.ac.uk>]

Thank-you for your help, Bill. I have got -stset- to identify all of the 
events now. There were several problems, all of which you had mentioned. 
I'm afraid my programming skills aren't all that good.

Best wishes
Sara

William Gould, Stata wrote:

> Sara Mottram <s.mottram@cphc.keele.ac.uk> writes, 
>   
> > I am having some difficulty with -stset-. I'm almost certain that the 
> > fault lies with my data, as this same command has worked before in a 
> > similar dataset. However, I wonder if anyone could give me an idea as to 
> > where I might start looking to find the problem.
> >
> > [...]
> >
> > [...] I know from a tabulation of the data that there are 734 
> > consultations, but when I use -stset- it identifies 730 events. One 
> > person consults at time 0, so I think this person is being ignored - I 
> > understand this. However, this still leaves three events that are 
> > unidentified.
> >
> > [...]
> >     
> And Sara included the following output:
>
>         -------------------------------------------------------------------
>         . stset cons_dt, id(surveyid) fail(kcons_post_3yr==1) origin(time 
>         > edateass) exit(time censor_date)
>
>                       id:  surveyid
>            failure event:  kcons_post_3yr == 1
>         obs. time interval:  (cons_dt[_n-1], cons_dt]
>         exit on or before:  time censor_date
>           t for analysis:  (time-origin)
>                   origin:  time edateass
>
>         ---------------------------------------------------------- 
>           16704  total obs.
>              28  obs. end on or before enter()
>         ---------------------------------------------------------- 
>           16676  obs. remaining, representing
>             742  subjects
>             730  failures in multiple failure-per-subject data
>          703420  total analysis time at risk, at risk from t =         0
>                                    earliest observed entry t =         0
>                                         last observed exit t =      1096
>         -------------------------------------------------------------------
>
>
> First, Sara, notice how Stata writes the time interval:
>
>         obs. time interval:  (cons_dt[_n-1], cons_dt]
>
> That is ( meaning open interval and ] meaning closed interval.  Hence, 
> a subject with the interval (0,0] makes no sense.  That subject failed 
> before he or she entered.
>
> Do you have other examples like this.  Do you, perhaps, have someone else 
> with interval (12,12] or (20,20]?  That would be the same story.
>
> Note that -stset- reported 
>              28  obs. end on or before enter()
>
> so Sara must have obs like (12,12] or (20,20], or she has more obvious 
> errors such as (20,12].
>
> Assuming the problems are all of the form (12,12] and (20,20], I would do 
> the following:
>
>         . replace censor_date = censor_date + .125
>
> and try again.  I'm assuming that Sara's dates are all integers and so 
> moving all the censoring dates forward just a little won't matter.
> There's nothing magic about .125; Sara could use .0625 or .03125 or even, 
> say .00390625.  Or .1, .01, .001, etc.  The only reason I don't use nice 
> numbers like .1, and .01 is that binary computers cannot store exactly 
> negative powers of 10, and so later, I cannot type things like 
>         . list if censor_date==12.1
>
> I have to type things like 
>         . list if censor_date==float(12.1)
>
> and I invariably forget, so I use negative powers of 2 to shift dates.
>
> Anyway, perhaps moving the end dates forward just a little will solve the
> problem.
>
> Or maybe not.  Sara has lots of dates in her files.  Quoting from the output 
> again:
>
>         obs. time interval:  (cons_dt[_n-1], cons_dt]
>         exit on or before:  time censor_date
>           t for analysis:  (time-origin)
>                   origin:  time edateass
>
> So we need to look at cons_dt as well.  And we need to look censor_date and
> edateass carefully, because Sara has multiple records per subject.
>
> I would do the following:
>
>         . sort surveyid cons_dt
>
>                                 // make sure dates are growing
>         . by surveyid: assert cons_dt > cons_dt[_n-1] if _n>1
>
>                                 // make sure censor_date is constant
>         . by surveyid: assert censor_date == censor_date[1]
>
>                                 // make sure edateass is constant
>         . by surveyid: assert edateass == edateass[1]
>
>                                 // make sure censor_date after enter date
>         . by surveyid: assert censor_date > cons_dt[1]
>
> -- Bill
> wgould@stata.com
> *
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>   
--
Sara Mottram	
Research Assistant: Biostatistics
Primary Care Musculoskeletal Research Centre
Primary Care Sciences
Keele University
Staffordshire, ST5 5BG
Tel:  	+44 (0) 1782 584711
Fax:  	+44 (0) 1782 583911
Email:	s.mottram@cphc.keele.ac.uk

*
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