Strange inconsistency between JMP(SAS) and R in median survival time estimationHi, everyone,
I came across the following strange inconsistency between R and JMP (or SAS) in median survival time estimation.
I understand that CI can be different because of different ways of doing it, but I never expected that median survival point estimation could be different. Does anyone here happen to know the actual technical reason? I tried to figure it out, but had no luck.
For the following simple dataset (event =1 means death, 0 means censor),
time | event | trt |
116 | 1 | 0 |
116 | 0 | 0 |
110 | 1 | 0 |
116 | 1 | 0 |
116 | 1 | 0 |
110 | 1 | 0 |
116 | 1 | 0 |
116 | 1 | 0 |
150 | 0 | 1 |
150 | 1 | 1 |
138 | 1 | 1 |
138 | 1 | 1 |
150 | 0 | 1 |
150 | 0 | 1 |
150 | 0 | 1 |
138 | 1 | 1 |
JMP cannot provide an estimate of median survival time for trt = 1 group while R can.
JMP output: (JMP 12.1, use analyze --> reliability and survival --> survival, Y = time, grouping = trt, cencor = event, censor code = 0 )
| Group | Median Time | ... |
| 0 | 116 | ... |
| 1 | . | ... |
R output: (code: library(survival); summary(survfit(Surv(time,event)~trt, data=data))$table)
| | ... | median | ... |
| trt=0 | ... | 116 | ... |
| trt=1 | ... | 150 | ... |
Thank you all in advance!
Re: Strange inconsistency between JMP(SAS) and R in median survival time estimationThanks a lot for the explanation! That makes sense!
Now I have to see if I can find a formula or something that gives a clear definition in SAS vs R, since I need to let R output the SAS result.
Thanks again!
Best,
Yan
Re: Strange inconsistency between JMP(SAS) and R in median survival time estimationI think what is going on is that there are too many right-censored observations to define the median. For the 8 observations in the trt=1 group, 3 are failures at t=138, 1 is a failure at t=150, and half (4) of them are right-censored at t=150. Since the median of 8 values is the average of the 4th and 5th values, we would need to average 150 and some (unknown) value larger than 150. As a result, the Survival platform is reporting a missing value for the Median Time.
As to why R gives a value, I can only speculate, but my guess is that since the 4th ordered value is not censored and the 4th observation out of 8 would be the 0.50 quantile, R is reporting the value of the 4th observation as the median survival time. Note that if you change one more t=150 observation (row 10) to be right-censored, R reports the median for trt=1 to be missing (NA).
Hope this helps,
Michael
