The LIFETEST Procedure
Computer Resources
The data are first read and sorted into strata. If the data are originally sorted by failure time and censoring state, with smaller failure times coming first and event values preceding censored values in cases of ties, the data can be processed by strata without additional sorting. Otherwise, the data are read into memory by strata and sorted.
Memory Requirements
For a given BY group, define the following:
![\begin{eqnarray*} N & & \mbox{the total number of observations} \\[0.05in] V & & \mbox{the number of STRATA variables} \\[0.05in] C & & \mbox{the number of covariates listed in the TEST statement} \\[0.05in] L & & \mbox{total length of the ID variables in bytes} \\[0.05in] S & & \mbox{number of strata} \\[0.05in] n & & \mbox{maximum number of observations within strata} \\[0.05in] b & & 12 + 8C + L \\[0.05in] m1 & & (112 + 16V) \times S \\[0.05in] m2 & & 50 \times b \times S \\[0.05in] m3 & & (50 + n) \times (b + 4) \\[0.05in] m4 & & 8(C + 4)^2 \\[0.05in] m5 & & 20N + 8S \times (S + 4) \\ \end{eqnarray*}](images/statug_lifetest0349.png)
The memory, in bytes, required to process the BY group is at least
![\[ m1 + \max (m2,m3) + m4 \]](images/statug_lifetest0350.png)
The test of equality of survival functions across strata requires additional memory (m5 bytes). However, if this additional memory is not available, PROC LIFETEST skips the test for equality of survival functions and finishes the other computations. Additional memory is required for the PLOTS= option. Temporary storage of 16n bytes is required to store the product-limit estimates for plotting.