# pwexpfit: Piecewise exponential survival estimation In keaven/nphsim: Non proportional hazards sample size and simulation

## Description

Computes survival function, density function, -2*log-likelihood based on input dataset and intervals for piecewise constant failure rates. Initial version assumes observations are right censored or events only.

## Usage

 ```1 2``` ```pwexpfit(Srv = Surv(time = Ex1delayedEffect\$month, event = Ex1delayedEffect\$evntd), intervals = array(3, 3)) ```

## Arguments

 `Srv` input survival object (see `Surv`; note that only 0=censored, 1=event for `Surv` `intervals` Vector containing positive values indicating interval lengths where the exponential rates are assumed. Note that a final infinite interval is added if any events occur after the final interval specified.

## Value

A matrix with rows containing interval length, estimated rate, -2*log-likelihood for each interval.

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22``` ```# use default arguments for delayed effect example dataset (Ex1delayedEffect) rateall <- pwexpfit() rateall # Estimate by treatment effect rate1 <- with(subset(Ex1delayedEffect,trt==1), pwexpfit(Surv(month,evntd))) rate0 <- with(subset(Ex1delayedEffect,trt==0), pwexpfit(Surv(month,evntd))) rate1 rate0 rate1\$rate/rate0\$rate # chi-square test for (any) treatment effect (8 - 4 parameters = 4 df) pchisq(sum(rateall\$m2ll)-sum(rate1\$m2ll+rate0\$m2ll), df = 4, lower=F) # compare with logrank survdiff(formula = Surv(month, evntd) ~ trt) # simple model with 3 rates same for each for 3 months, # different for each treatment after months rate1a <- with(subset(Ex1delayedEffect,trt==1), pwexpfit(Surv(month,evntd),3)) rate0a <- with(subset(Ex1delayedEffect,trt==0), pwexpfit(Surv(month,evntd),3)) rate1a\$rate/rate0a\$rate m2ll0 <- rateall\$m2ll[1]+rate1a\$m2ll[2]+rate0a\$m2ll[2] m2ll1 <- sum(rate0\$m2ll)+sum(rate1\$m2ll) # as a measure of strength, chi-square examines improvement in likelihood pchisq(m2ll0-m2ll1,5,lower=FALSE) ```

keaven/nphsim documentation built on July 2, 2018, 1:10 a.m.