--- Giulio Rizzoli <[email protected]> wrote:
> Dear experts I have these medical data.
> that i have pooled from the literature. They concern
> survival of
> medical devices biologic or mechanic.
> at-risk are the unit at risk.
> Survival are the % in use at stated time.
>
> Time at-risk-bio at-risk-mec Survival-bio
> Survival-mec
> 0 477 391 100 100
> 1 362 286 86.5 86.9
> 5 249 186 73.6 73.5
> 10 94 84 62 60.2
> 15 29 22 46.7 47.8
>
> I would like to know if it is possible and how to
> calculate
> the 95% Confidence Interval of the Survival
> difference.
This is difficult because the number of censored
observations impact the accuracy of these estimates. A
naive estimate can be obtained from -prtest-. For
example, for t=15:
. prtesti 29 .467 22 .478
Two-sample test of proportion x:
Number of obs = 29
y: Number of obs = 22
Variable Mean Std. Err. z P>z
[95% Conf. Interval]
x .467 .0926452 .2854187
.6485813
y .478 .1064971 .2692695
.6867305
diff -.011 .1411551
-.287659 .265659
under Ho: .14114 -0.08 0.938
Ho: proportion(x) - proportion(y) = diff = 0
Ha: diff < 0 Ha: diff != 0 Ha:
diff > 0
z = -0.078 z = -0.078 z =
-0.078
P < z = 0.4689 P > z = 0.9379 P > z
= 0.5311
That is:
diff= -.011 95%C.I.= -0.287659, 0.265659
Best,
Ricardo.
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