# Thread: Median Survival CI - SPSS vs Stata

1. ## Median Survival CI - SPSS vs Stata

I've been generating some Kaplan Meier survial curves and the related stats. SPSS and Stata give me the same Median survival but very different 95% confidence intervals. Could someone explain the disparity for me?

Group 1
SPSS: Median 60, SE 5.5, CI 49.2-70.8
Stata: Median 60, SE 5.5, CI 51-94

Group 2
SPSS: Median 39, SE 5.8, CI 27.6-50.4
Stata: Median 39, SE 5.8, CI 27-78

Group 3
SPSS: Median 116, SE 14.6, CI 87.3-144.7
Stata: Median 116, SE 14.6, CI 66-136

Many thanks!

2. ## Re: Median Survival CI - SPSS vs Stata

Hmm, I haven't run survival analyses in either of those programs before. The SPSS calculation seems to be the traditional +/- 1.96(SE), while the STATA seems to not be symmetrical around the median. Perhaps meaning it is using an exponentiate'd value maybe.

Can you provide the output as well. I was initially thinking it could be one is using confidence interval and the other is using prediction interval, but I don't think the asymmetry would exist.

3. ## Re: Median Survival CI - SPSS vs Stata

Stata outputs this:

. stci if MRI_in_30d==0,by(First_MRI_SEP)

failure _d: RIP == 1
analysis time _t: FUpDurWeeks

Code:
|    no. of
First_MRI_~P |  subjects         50%     Std. Err.     [95% Conf. Interval]
-------------+-------------------------------------------------------------
Equ |        33          60      5.507446           51         94
Prog |        15          39      5.796551           27         78
Sta+Reg |        14         116      14.61789           66        136
-------------+-------------------------------------------------------------
total |        62          71      11.10534           57         94

Not sure if that's what you were after....

I did some more digging and found this:
http://www.stata.com/manuals13/ststci.pdf

The methodology for working out the CI is over my head but I think it's safe to say that it's a more realistic method than just +/-1.96 SE?

4. ## Re: Median Survival CI - SPSS vs Stata

It has been some time since I ran a survival model, it kind of seemed like the link was for K-M based values and it grabbed the interval value closest to the respective percentile. Sorry I can't be of more help off the top of my head.

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