# Re: [R] odd dnorm behaviour (?)

From: Charles C. Berry <cberry_at_tajo.ucsd.edu>
Date: Mon, 07 Jul 2008 22:39:06 -0700

On Tue, 8 Jul 2008, Mike Lawrence wrote:

> #Let's try this again! This time the code is more sensible (curve range, same
> sd value).
>
> # Quick one hopefully. Shouldn't dnorm be returning the pdf?

No. See the documentation

?dnorm

especially what it says under 'Value'

> Last time I
> # checked,
> # a probability shouldn't be greater than 1 as produced by:
>
> curve(dnorm(x,0,.1),from=-.5,to=.5)

I do not see a probability that is greater than one here:

> integrate(function(x) dnorm(x,0,.1),lower=-Inf,upper=Inf)
1 with absolute error < 6.1e-07

... looks like 1 to me.

>
> #Shouldn't I be getting an axis more like that produced by:
>
> f=function(x,m,s){
> y=rep(NA,length(x))
> for(i in 1:length(x)){
> y[i]=integrate(
> dnorm
> , upper=x[i]+sqrt(.Machine\$double.eps)
> , lower=x[i]-sqrt(.Machine\$double.eps)
> , mean=m
> , sd=s
> ) \$value
> }
> return(y)
> }
> curve(f(x,0,.1),from=-.5,to=.5)

Well, what does this suggest?

> integrate(function(x) f(x,0,.1),lower=-Inf,upper=Inf)
2.980232e-08 with absolute error < 2.6e-10
>
> sqrt(.Machine\$double.eps)*2

[1] 2.980232e-08

Well, the posting guide does ask you to do some homework before posting.

Nowadays, it is as easy. Google 'wiki probability density function' or look in the reference in ?dnorm.

>
> #Mike
>
> --
> Mike Lawrence
> Graduate Student, Department of Psychology, Dalhousie University
>
> www.memetic.ca
>
> "The road to wisdom? Well, it's plain and simple to express:
> Err and err and err again, but less and less and less."
> - Piet Hein
>
> ______________________________________________
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>

```Charles C. Berry                            (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry_at_tajo.ucsd.edu	            UC San Diego
```
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901

R-help_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Tue 08 Jul 2008 - 05:41:47 GMT

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