Re: st: using srvyset with survival analysis

[Sharon Minnick <slminnick@ucdavis.edu>]

Hello,

Many thanks to Jeff Pitblado for his quick reply to my original query 
and apologies that it has taken me so long to get back, but I have been 
traveling. In my original message I explained that I get an error that 
there are "no observations; stset and subpop() option identify disjoint 
subsets of the data" when I try to use svy: stcox in Stata10. I have 
simplified my model to attempt to get svy to work with stcox but I 
continue to get the "no observations" error as before. Performing the 
checks that Jeff suggested indicate that there are no missing values so 
I am at a loss as to what the problem is. My simplified svyset command 
now includes only 1 level, sampling of individuals but stratified by a 
zone variable which is constant and nonmissing for all subjects (as 
reported by stvary and manual checks), but I get the same error with all 
attempts to svyset:

. svyset id, strata(fixedzone) vce(linearized) singleunit(certainty)

pweight: <none>
VCE: linearized
Single unit: certainty
Strata 1: fixedzone
SU 1: id
FPC 1: <zero>

The stset command looks like:

. stset stdate_b, id(id) failure(anysc) time0(stdate_a) exit(anysc==1 
time d(30jun2001)) origin(time stdate_a)

Here is the result from stdes:
. stdes
failure _d: anysc
analysis time _t: (stdate_b-origin)
origin: time stdate_a
exit on or before: anysc==1 time d(30jun2001)
id: id

|-------------- per subject --------------|
Category total mean min median max
------------------------------------------------------------------------------ 
no. of subjects 1650
no. of records 4692 2.843636 1 3 10
(first) entry time 0 0 0 0
(final) exit time 569.5158 86 611 860
subjects with gap 3
time on gap if gap 649 216.3333 205 220 224
time at risk 939052 569.1224 86 611 860
failures 143 .0866667 0 0 1
------------------------------------------------------------------------------ 

(the gaps resulted from dropping observations in which subjects had 
moved from their
original locations, 3 of them moved back after some time; the first 
subjects were enrolled in 1999 and I need to end this period of analysis 
at June 30, 2001 for a reason).

The result from "svydes agegrp1" indicates that I have a minimum of 111 
units per stratum and a minimum of 1 observation included per unit and 0 
units omitted and 0 observations with missing data.

As far as I can tell, I don’t have any missing data that would be 
causing the error,
(and I've temporarily changed the single unit option to certainty to 
avoid missing data even though I don't think I have any) but here is 
what happens:

. svy: stcox agegrp1

no observations;
stset and subpop() option identify disjoint subsets of the data
r(2000);

Thanks in advance for any further help!

Sharon Minnick
UC Davis


Jeff Pitblado, StataCorp LP wrote:

> Sharon Minnick <slminnick@ucdavis.edu> is working with survival data 
> from a
> complex survey, and is having trouble with -svy: stcox-:
>
>
> > In Stata10, I have been attempting to analyze my survival data with 
> > cox regression while accounting for the sampling design and I always 
> > get this error:
> >
> > the stset ID variable is not nested within the final stage sampling unit
> >
> > Originally I thought this might be because some of our subjects moved 
> > during the study and so they change strata and cluster from where 
> > they were originally sampled. But I just tried creating a temporary 
> > variable that holds the strata constant for each subject and dropped 
> > the cluster variables and I still get the same error.
> >
> > So my srvyset code is:
> >
> > svyset _n, strata(staticzone) vce(linearized) singleunit(missing)
> >
> > and the cox regression code is:
> >
> > svy: stcox agegrp male
> >
> > If I change the svyset code to use my subject ID variable instead of 
> > _n, then I get this error:
> >
> > no observations;
> > stset and subpop() option identify disjoint subsets of the data
> >
> > which I don't understand since I am not using a subpop option.
> The -svy- prefix, when used with -stcox- or -streg-, requires that 
> subjects
> with multiple records be contained within the final stage clusters. Thus
> subjects are not allowed to belong to more than one cluster.
>
> Although Sharon did not show us the -stset- command she used, it 
> appears that
> it looked something like
>
> . stset time, id(subject) failure(failed) ...
>
> where 'time', 'subject', and 'failed' represent the names of Stata 
> variables
> Sharon used to -stset- her data.
>
> Given the above -stset-, Sharon should use the 'subject' variable 
> instead of
> '_n' to identify the final stage units. If Sharon's has data from a
> single-stage survey design, this means that the -svyset- command 
> should look
> like
>
> . svyset subject, strata(staticzone) vce(linearized) singleunit(missing)
>
> Using '_n' implies that the records were sampled in the first stage,
> which cannot be true given the above -stset-.
>
> After changing her -svyset- to use the 'subject' variable, Sharon was
> presented with the following error message:
>
> no observations;
> stset and subpop() option identify disjoint subsets of the data
>
> This indicates that -svy- was left with 'no observations' after removing
> observations with missing values and checking for subpopulation
> specifications.
>
> In addition to the -subpop()- option, -svy- uses the following options of
> -stset- to identify the subpopulation:
>
> if(exp)
> ever(exp)
> never(exp)
> after(exp)
> before(exp)
>
> Without Sharon's dataset, we can't say definitively what is going on.
> However, comparing the results from the following two commands
>
> . stset
> . svydes agegrp male
>
> should indicate how many observations are being dropped because of 
> missing
> values. The 'no observations' error message will result if the 
> subpopulation
> specification identifies only observations that contain missing values 
> in the
> variables of interest (that is: time, subject, failed, staticzone, 
> agegrp, and
> male).
>
> --Jeff
> jpitblado@stata.com
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