Calculating KM curves

It is worth revisiting the calculation of the Kaplan—Meier curve following on from the discussion of the hazard rate, in order to see, firstly, how censoring is accounted for, and secondly, how the two are linked in terms of the calculation. 

Now to the calculation of the Kaplan-Meier survival probabilities. Using the above example, the estimated probability of surviving beyond month 1 is 

1 — (7/1000) = 0.9993. The probability of surviving beyond month 2 is the probability of surviving beyond month 1 x the probability of surviving beyond month 

Note also here that the numbers 1000, 983 and 960 are the numbers of patients alive in the trial, in that group, at the start of months 1, 2 and 3 respectively, the risk sets. 

This calculation in relation to both the hazard rate and the survival probabilities has been undertaken at intervals of one month. In practice we use intervals which correspond to the unit of measurement for the endpoint itself, usually days, in order to use the total amount of information available. 

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