[R] Confidence Interval for SMR

[Terry Therneau]

> Hello,
> 
> I am looking for a function which allows to calculate the confidence
> interval for a standard mortality ratio. I do have vectors with the
> number of observed and expected death. Has anybody a hint where to
> look?
> Best,
> Stefan

 We find the following function useful for an SMR
    cipoisson(observed, expected)
    
    	Terry Therneau
    	Mayo Clinic
    
"cipoisson" <- 
function(k, time = 1, p = 0.95, method = c("exact", "anscombe") ) {
	nn <- max(length(k), length(time), length(p))
	if(nn > 1) {
		k <- rep(k, length = nn)
		time <- rep(time, length = nn)
		p <- rep(p, length = nn)
	}
	p <- (1 - p)/2  #two sided

	method <- match.arg(method)
	if(method == "exact") {
		dummy1 <- ifelse(k == 0, 1, k)
		#avoid an error message of qgamma
		lower <- ifelse(k == 0, 0, qgamma(p, dummy1))
		upper <- qgamma(1 - p, k + 1)
	}
	else if(method == "anscombe") {
		# anscombe's method
		upper <- (sqrt(k + 7/8) - qnorm(p)/2)^2
		lower <- (sqrt(k - 1/8) + qnorm(p)/2)^2
	}
	else stop("Invalid method")
	if(nn == 1)
		c(lower = lower, upper = upper)/time
	else cbind(lower = lower, upper = upper)/time
}