R help - Dec 2006 - counting process and output of survfit

Hello,

I put my data in conditional counting process format basically by adding
a stratum variable to distinguish between observations.

I fit a model and then used survfit function. These are the first five lines
of
output.

  time n.risk n.event survival  std.err lower 95% CI upper 95% CI
     849     11       1 9.22e-01 7.53e-02     7.85e-01     1.000000
    3025     28       1 8.91e-01 7.87e-02     7.50e-01     1.000000
    3089     28       1 8.62e-01 8.14e-02     7.16e-01     1.000000
    4691     37       1 8.39e-01 8.23e-02     6.92e-01     1.000000
    5295     43       1 8.21e-01 8.26e-02     6.74e-01     0.999566


My questions:

1. How does it determine n.risk at time t?
2. How does it calculate the survival at the end of time t?

thank you!

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