# [R] adding one line to a plot

Jannis bt_jannis at yahoo.de
Tue May 25 20:28:24 CEST 2010

```Your line does show up, but the x values start at ~6 and the y values are incredibly small, so it is masked by the y=0 line. You probably have to check your calculations!

--- Dimitri Liakhovitski <dimitri.liakhovitski at gmail.com> schrieb am Mo, 24.5.2010:

> Von: Dimitri Liakhovitski <dimitri.liakhovitski at gmail.com>
> Betreff: [R] adding one line to a plot
> An: r-help at r-project.org
> Datum: Montag, 24. Mai, 2010 16:50 Uhr
> Hello!
>
> I am running a very simple mini Monte-Carlo below using the
> function
> tstatistic (right below this sentence):
>
> tstatistic = function(x,y){
>     m=length(x)
>     n=length(y)
>     sp=sqrt( ((m-1)*sd(x)^2 +
> (n-1)*sd(y)^2)/(m+n-2) )
>     t=(mean(x)-mean(y))/(sp*sqrt(1/m+1/n))
>     return(t)
> }
>
> alpha=.1; m=10; n=10 # sets alpha, m, n - for run 1
> N=10000 # sets the number of simulations
> n.reject=0 # counter of num. of rejections
> tstat<-vector()
> for (i in 1:N){
>     x = rnorm(m,mean=10,sd=2) # simulates xs
> from population 1
>     y=rexp(n,rate=1/10)
>     t = tstatistic(x,y) # computes the t
> statistic
>     tstat = c(tstat,t)
>     if (abs(t)>qt(1-alpha/2,n+m-2))
>     n.reject=n.reject+1 # reject if |t|
> exceeds critical pt
> }
> true.sig.level=n.reject/N # est. is proportion of
> rejections
> true.sig.level
>
> And then I would like to plot the observed t values (in the
> vector
> "tstat") against the values of the t density with df=18.
> Somehow, my t
> denstity (code line 2 below) does not show up:
> plot(density(tstat),xlim=c(-5,8),ylim=c(0,.4),lwd=3)
> lines(x,dt(x,df=18))
> legend(4,.3,c("exact","t(18)"),lwd=c(3,1))
>
> Thanks a lot!
> D.
>
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