On 13-11-30 4:37 PM, Loyack, Eric wrote:
I am using R Studio and writing code to determine p-values for statistical
t-tests. Code is below. When I print out the values in the loop for
PTMpvalMeans they are correct, but when I store them the 10th one
overwrites the 11th. Can someone tell me what I am doing wrong? Thanks in
advance, Eric
It looks like a fairly obscure case of FAQ 7.31. It's a bad idea to use
0.1 as a loop increment, because fractions like that can't be
represented exactly, and rounding error accumulates.
Duncan Murdoch
rho <- 0.0
PTMpvalMeans <- numeric(11)
while (rho < 1.0){
x1 <- rnorm(25,0,1)
x2 <- rnorm(25,0,1)
y1 <- rnorm(25,0,1)
y2 <- rnorm(25,0,1)
w1 <- rho*x1 + sqrt(1-rho^2)*y1
w2 <- rho*x2 + sqrt(1-rho^2)*y2
controlPretest <- 50 + 10*x1
groupPretest <- 50 + 10*x2
controlPosttest <- 60 + 10*w1
groupPosttest <- 70 + 10*w2
postTestMethod <-
t.test(controlPosttest,groupPosttest,mu=0,alternative="two.sided")
PTMpval <- postTestMethod$p.value
for (i in 1:999){
x1 <- rnorm(25,0,1)
x2 <- rnorm(25,0,1)
y1 <- rnorm(25,0,1)
y2 <- rnorm(25,0,1)
w1 <- rho*x1 + sqrt(1-rho^2)*y1
w2 <- rho*x2 + sqrt(1-rho^2)*y2
controlPretestLoop <- 50 + 10*x1
groupPretestLoop <- 50 + 10*x2
controlPosttestLoop <- 60 + 10*w1
groupPosttestLoop <- 70 + 10*w2
controlPretest <- c(controlPretest,controlPretestLoop)
groupPosttest <- c(groupPosttest,groupPosttestLoop)
groupPretest <- c(groupPretest,groupPretestLoop)
controlPosttest <- c(controlPosttest,controlPosttestLoop)
postTestMethodLoop <-
t.test(controlPosttestLoop,groupPosttestLoop,mu=0,alternative="two.sided")
PTMpvalLoop <- postTestMethodLoop$p.value
PTMpval <- c(PTMpval,PTMpvalLoop)
}
PTMpvalMeans[rho*10 + 1] <- mean(PTMpval)
print(rho)
print(PTMpvalMeans[rho*10 + 1])
rho <- rho + 0.1
}
PTMpvalMeans
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