置换测试，获得NaN结果

Question

我们在标记为release的列中有数据，其中有两个选项，release和non release。我们正在尝试进行置换测试，但是结果返回为Na。

dat$frelease <- factor(recode(dat$release, `nonrelease`="nonrelease", `release`="release"))
dat$release <- as.factor (dat$release) 
dat$isrelease<- as.numeric (dat$release) 
dat
dat$frelease <- factor(recode(dat$release, `1`="nonrelease",      `2`="release"))

(obsdiff <- mean(dat[dat$frelease == "Nonrelease", "damage"]) -     mean(dat[dat$frelease == "Nonrelease", "damage"]))

nreps <- 50
rdiffs <- numeric(nreps)
for(i in 1:nreps){
  rand.damage <- sample(dat$damage, size = length(dat$damage),     replace = FALSE)
  rdiffs[i] <- mean(rand.damage[dat$frelease == "release"]) -     mean(rand.damage[dat$frelease == "nonrelease"])

}     均值（rdiffs> = obsdiff

This is the result when being but into R
> mean(rdiffs >= obsdiff)
[1] NA
> nreps <- 50
> rdiffs <- numeric(nreps)
> (obsdiff <- mean(dat[dat$frelease == "Nonrelease", "damage"]) - mean(dat[dat$frelease == "Nonrelease", "damage"]))
[1] NaN
> nreps <- 50
> rdiffs <- numeric(nreps)
> for(i in 1:nreps){
+   rand.damage <- sample(dat$damage, size = length(dat$damage), replace = FALSE)
+   rdiffs[i] <- mean(rand.damage[dat$frelease == "release"]) - mean(rand.damage[dat$frelease == "nonrelease"])
+ }
> mean(rdiffs >= obsdiff)
[1] NA