# Probability calculation in survival analysis

I have 13 similar independent units and I need to calculate the probability at least 6 of them to survive for time less than 1.10.

From a table with the Kaplan-Meir estimates I get the following;

Time          1.05614 , 1.31581
number at risk  19   ,   18
number failed 1,1
survival Prob. 0.474771 , 0.448395


I have omitted the previous and the next values because Time=1.10 is between those two values and P(T<1.10)=F(1.10)=F(1.05614)=1-survival prob.=1-0.474771= 0.525229

Which is the probability for 1 unit to survive for less than 1.10 hours? I don't know to continue from that point.

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 What does "at least $6$ of them to survive for time less than $1.10$" mean? Usually, questions in survival analysis, reliability analysis and the like would ask for the probability that "at least $6$ of them survive till time $1.10$" or "at least $6$ of them survive past time $1.10$", that is, $4$ or fewer items fail before $1.10$. – Dilip Sarwate Feb 12 '12 at 14:02 @Dilip Probably my translation to English was unsuccessful. I'll try to explain it. What I want is the probability of at least 6 of the 13 units to have time of life less than 1.10. – user9149 Feb 12 '12 at 14:30