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Re: st: comparing 25th percentile survival time between two race groups


From   Haena Lee <hannahlee419@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: comparing 25th percentile survival time between two race groups
Date   Fri, 28 Sep 2012 18:04:52 -0500

Thank you Steve, That's really good point to look at the graph
especially for the failures.

According to ranksum, I was not able to find an option for 25th percentiles.
When I was comparing the differences of median survival time between
African American and White across 10 different regions in US, I used
this following command;

. sort UNOS
. by UNOS: ranksum d_enrollment, by(race_all)

In this case, I didn't need to let Stata know it is median survival
time because I guess median was default. However my question is how I
can particularly tell Stata that it is to test 25th percentiles this
time. Any help? Thanks much.

On Fri, Sep 28, 2012 at 4:26 PM, Steve Samuels <sjsamuels@gmail.com> wrote:
> Dear Haena:
>>
>> On Sep 28, 2012, at 2:43 PM, Haena Lee wrote:
>>
>> Hi Listers,
>> I have tried to produce the median survival time and its 95% CI with
>> renal transplant data. "Stci" command, however, did not produce any
>> median values. So I generated the graph of median survival time by
>> race (African American vs. White) and the graph showed it only reached
>> to the length of 25% in the course of follow-up, indicating both of
>> groups have died not even near 50% (median), but mostly near the
>> length of 25% tile in the course of follow-up.
>>
> What you observe can be better phrased as saying that followup was not
> long enough to observe 50% of failures.
>
>> 1. Given this, I attempted to produce 25th percentile survival time.
>> However I am struggling with comparing the difference of 25th
>> percentile survival time between African American and White across 10
>> regions in US. The command that I used:
>> sort UNOS
>> by UNOS: stci , by(race_all) p(25)
>
>>
>> I was trying to use "ranksum" but it doesn't have an option for
>> testing differences of other percentiles besides median. Which command
>> should I use?
>
> That's a correct command. It tests for any difference where survival
> in one group is better, not just differences in medians.
>>
>> 2. In comparing the median to the 25 percentile, there are some
>> regions where the length to 25% tile is significantly longer than
>> median. In this case, what would you guys do? If you would choose to
>> use median survival time, then what else could I do in order not only
>> to produce the median survival time and 95% CI, but to test the
>> difference of median survival time between groups instead stci (since
>> it didn't produce in previous analysis)?
>
> Your observation is impossible in theory; to persuade us, you'd need to
> follow the advice in Statalist FAQ (3.3) and show us the commands and
> results that lead you to this observation.
>
> If I had to guess, I'd say that you looked at the Kaplan-Meier survival
> curve, not at -stci- results. A survival curve shows the proportion of
> people who haven't had the event at a time, not the proportion who have
> failed. Thus the point corresponding to the 25% mark on the y axis is where
> 25% of people haven't failed and 75% have. In other words it is the 75th
> percentile for failure. To see the failure time percentiles, the command
> is:
>
> *******************
> sts graph, failure
> *******************
>
> You might benefit from reading a good text on survival data, e.g. Cleves
> et al. (2010).
>
> Reference: Cleves, Mario, William Gould, Roberto Gutierrez, and Yulia
> Marchenko. 2010. An Introduction to Survival Analysis Using Stata, Third
> Edition. College Station, Tex: Stata Press.
>
> Steve
>
>> Thank you so much!
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-- 
=====================
Haena Lee
Ph.D Student
Sociology Department
The University of Chicago
312 - 405 - 3223
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