R help - Oct 2010 - sampling from normal

Hello
If i want to resample from the lower tail of normal distribution , are these
commands equivelant??
 lower tail :qnorm(runif(n,0,pnorm(a))) if a is a lower tail bound
or
 lower tail:qnorm(p*runif(n)) if p is the probability of each interval(the
observations are divided to intervals)
 
Regards


      
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Hi Solafah,

You are right that two commands are equivalent when p= pnorm(a). You can
check the results by following codes.
n <- 5
a <- -1
set.seed(123456)
qnorm(runif(n,0,pnorm(a)))
p <- pnorm(a)
set.seed(123456)
qnorm(p*runif(n)) 

Anyway, the elements of the lower tail are not chosen equally by this
method. I may try another method. Such like:
s1 <- rnorm(10000)
n <- 5
a <- -1
sample(s1[s1<a],n)


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