My data contain multiple records per subject.
Subjects are at risk once a year, starting the first of June, for a period of approximately 32 hours (i.e., they are at risk less than two days in any given year).
Some subjects were at risk repeatedly over twenty years--that is, in the first two days of June, several different years between 1986 and 2007.
What is the correct way to invoke -stset- in such a situation?
The output below shows that I am not doing it right. In spite of my use of the "origin" and "id" options, -stset- assigns the same value for _origin to each subject; issues the warning, "3485 obs. begin on or after (first) failure"; and drops those observations from analysis. This can be seen in the partial listing below, in that most of the _t and _t0 values for each subject are missing.
Before attempting formal analysis with a shared frailty model, I would like to generate a Kaplan-Meier curve that summarizes the information about time to event across all subjects and all 20 years. Thus I would like the time axis of the plot to range from zero to about 33 hours. I do not want the time axis to reflect the stretch of time from 1986 to 2007, but rather just the period of *elapsed* time, less than two days, when subject are at risk.
But the -sts- command below produces a time axis that stretches over 20 years, uninformative.
I apologize for the basic error that I must be making. Thanks to whoever can set me straight.
Details below.
. des finTime FinYes June1thisYear
storage display value
variable name type format label variable label
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
finTime float %9.0g Time failed or censored, a Stata date variable (unit=days)
FinYes float %9.0g 0: not finished; 1: finished
June1thisYear float %dM_d,_CY When subject starts to be at risk this year
. local onehour=1/24
. stset finTime, id(nNAME) failure(FinYes) origin(time June1thisYear) scale(`onehour')
id: nNAME
failure event: FinYes != 0 & FinYes < .
obs. time interval: (finTime[_n-1], finTime]
exit on or before: failure
t for analysis: (time-origin)/.0416667
origin: time June1thisYear
------------------------------------------------------------------------------
8282 total obs.
3485 obs. begin on or after (first) failure
------------------------------------------------------------------------------
4797 obs. remaining, representing
3956 subjects
2933 failures in single failure-per-subject data
1.71e+07 total analysis time at risk, at risk from t = 0
earliest observed entry t = 0
last observed exit t = 184111.8
.
. list Year finTime FinYes nNAME June1thisYear _* if nNAME < 5
+----------------------------------------------------------------------------------------------+
| Year finTime FinYes nNAME June1thisY~r _st _d _origin _t _t0 |
|----------------------------------------------------------------------------------------------|
1. | 1986 9649.137 1 1 June 1, 1986 1 1 9648 27.281228 0 |
2. | 1987 10014.33 0 1 June 1, 1987 0 . 9648 . . |
3. | 1988 10380.33 0 1 June 1, 1988 0 . 9648 . . |
4. | 1989 10745.23 1 1 June 1, 1989 0 . 9648 . . |
5. | 1990 11110.12 1 1 June 1, 1990 0 . 9648 . . |
|----------------------------------------------------------------------------------------------|
6. | 1992 11841.33 0 1 June 1, 1992 0 . 9648 . . |
7. | 1993 12206.33 0 1 June 1, 1993 0 . 9648 . . |
8. | 1994 12571.23 1 1 June 1, 1994 0 . 9648 . . |
9. | 1995 12936.33 0 1 June 1, 1995 0 . 9648 . . |
10. | 1996 13302.33 0 1 June 1, 1996 0 . 9648 . . |
|----------------------------------------------------------------------------------------------|
11. | 1997 13667.33 0 1 June 1, 1997 0 . 9648 . . |
12. | 1998 14032.33 0 1 June 1, 1998 0 . 9648 . . |
13. | 1999 14397.33 0 1 June 1, 1999 0 . 9648 . . |
14. | 2000 14763.33 0 1 June 1, 2000 0 . 9648 . . |
15. | 2001 15128.33 0 1 June 1, 2001 0 . 9648 . . |
|----------------------------------------------------------------------------------------------|
16. | 2002 15493.33 0 1 June 1, 2002 0 . 9648 . . |
17. | 2003 15858.33 0 1 June 1, 2003 0 . 9648 . . |
18. | 2005 16589.33 0 1 June 1, 2005 0 . 9648 . . |
19. | 2006 16954.33 0 1 June 1, 2006 0 . 9648 . . |
20. | 2007 17319.33 0 1 June 1, 2007 0 . 9648 . . |
|----------------------------------------------------------------------------------------------|
21. | 1994 12571.33 0 2 June 1, 1994 1 0 12570 31.945287 0 |
22. | 1997 13666.98 1 2 June 1, 1997 1 1 12570 26327.463 31.945287 |
23. | 1998 14031.98 1 2 June 1, 1998 0 . 12570 . . |
24. | 2001 15127.98 1 2 June 1, 2001 0 . 12570 . . |
25. | 2005 16588.9 1 3 June 1, 2005 1 1 16588 21.656233 0 |
|----------------------------------------------------------------------------------------------|
26. | 2006 16953.91 1 3 June 1, 2006 0 . 16588 . . |
27. | 2007 17318.88 1 3 June 1, 2007 0 . 16588 . . |
28. | 2005 16589.22 1 4 June 1, 2005 1 1 16588 29.390601 0 |
+----------------------------------------------------------------------------------------------+
. sts
failure _d: FinYes
analysis time _t: (finTime-origin)/.0416667
origin: time June1thisYear
id: nNAME
Jacob A. Wegelin
Assistant Professor
Department of Biostatistics
Virginia Commonwealth University
730 East Broad Street Room 3006
P. O. Box 980032
Richmond VA 23298-0032
U.S.A.
E-mail: [email protected]
URL: http://www.people.vcu.edu/~jwegelin
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