Windows 7, R 2.12.1
Colleagues,
I am trying to understand the n.for.2means function. The code below is a copy
of the function (renamed to n.for.2means.js). I have inserted a single line of
code towards the bottom of the function which uses the cat function to print
the value of n1. You will note the value (preceded by stars) is printed as 1.
The function (1) prints a lot of output without any instructions in the
function to print anything (other than the cat statement I added), and when it
prints (2) reports the value of n1 to be 2!.
I have two questions, (i) how is the function printing when there is no code to
print and (ii) how is n1 which equals 1 being reported as 2? I suspect there is
something fundamental about R that I don't know.
Thank you for the help.
John
library(epicalc)
n.for.2means.js <- function (mu1, mu2, sd1, sd2, ratio = 1, alpha = 0.05, power
= 0.8)
{
n1 <- (sd1^2 + sd2^2/ratio) * (qnorm(1 - alpha/2) - qnorm(1 - power))^2/(mu1
- mu2)^2
n1 <- round(n1)
n2 <- ratio * n1
if (length(alpha) == 1) {
alpha1 <- NULL
}
else {
alpha1 <- alpha
}
if (length(power) == 1) {
power1 <- NULL
}
else {
power1 <- power
}
if (length(ratio) == 1) {
ratio1 <- NULL
}
else {
ratio1 <- ratio
}
table1 <- cbind(mu1, mu2, sd1, sd2, n1, n2, alpha1, power1,
ratio1)
colnames(table1)[colnames(table1) == "alpha1"] <- "alpha"
colnames(table1)[colnames(table1) == "power1"] <- "power"
colnames(table1)[colnames(table1) == "ratio1"] <- "n2/n1"
table1 <- as.data.frame(table1)
cat("******n1*******=",n1,"\n")
returns <- list(mu1 = mu1, mu2 = mu2, sd1 = sd1, sd2 = sd2,
alpha = alpha, n1 = n1, n2 = n2, power = power, ratio =
ratio,
table = table1)
class(returns) <- c("n.for.2means", "list")
returns
}
n.for.2means.js(0,8,10/6,10/6)
John David Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
University of Maryland School of Medicine Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
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