[R] Probabilities greather than 1 in HIST

[David Winsemius]

On Mar 3, 2011, at 8:03 AM, jpmaroco wrote:

> Dear all,
> I am a newbie in R and could not find help on this problem. I am  
> trying to
> plot an histogram with probabilities in the y axis. This is the code  
> I am
> using:
>
> #TLC uniform
> n=30
> mi=1; mx=6
> nrep=1000
> xbar=rep(0,nrep)
> for (i in 1:nrep) {xbar[i]=mean(runif(n,min=mi,max=mx))}
> hist(xbar,prob=TRUE,breaks="Sturges",xlim=c(1,6),main=paste("n =",n),
> xlab="Média", ylab="Probabilidade")
> curve(dnorm(x,mean=mean(xbar),sd=sd(xbar)),add=TRUE,lwd=2,col="red")
>
> The problem is that I am getting greater than 1 probabilities in the  
> Y axis?
> Is there a way to correct this?

Despite the argument name, which I agree suggests that probabilities  
will be plotted, what is really described in the help page is that  
densities will be plotted, and densities may be greater than 1. You  
can suppress plotting of the y-axis, calculate the probabilities for  
each of the groups returned by hist,  and then use the axes function.

 > xhist <- hist(xbar,breaks="Sturges",plot=FALSE)
 > yhist <- xhist$counts/sum(xhist$counts)
 > yhist
  [1] 0.002 0.027 0.087 0.236 0.287 0.228 0.107 0.021 0.004 0.001



> Many thanks in advance.
> Joao
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT