R help - May 2007 - polygon error?

Hi.. I'm not sure why polygon returns an area above the standard normal
curve.

z <- pretty(c(-3,3), 100)
ht <- dnorm(z)
data <- data.frame(z=z, ht=ht)
zc <- 1.645
plot(data, type="l")
lines(data)
t <- subset(data, z>zc)
polygon(t, col="red")

Thanks,
Lance


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polygon expects to be given the vertices, so if you want the area  
under the curve you'll want to include vertices at density=0

z <- pretty(c(-3,3), 100)
ht <- dnorm(z)
plot(z,ht, type="l")

zc <- 1.645
ht<-ht[z>zc]
z<-z[z>zc]

ht<-c(0,ht,0)
z<-c(z[1],z,z[length(z)])

polygon(z,ht,col='red')


On 26-May-07, at 7:34 AM, LL wrote:
> Hi.. I'm not sure why polygon returns an area above the standard  
> normal curve.
>
> z <- pretty(c(-3,3), 100)
> ht <- dnorm(z)
> data <- data.frame(z=z, ht=ht)
> zc <- 1.645
> plot(data, type="l")
> lines(data)
> t <- subset(data, z>zc)
> polygon(t, col="red")
>
> Thanks,
> Lance
>
>
> 	[[alternative HTML version deleted]]
>
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--
Mike Lawrence
Graduate Student, Department of Psychology, Dalhousie University

Website: http://myweb.dal.ca/mc973993
Public calendar: http://icalx.com/public/informavore/Public

"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

Ok.. I see that I did not completely define the polygon. The code below works
fine

z <- pretty(c(-3,3), 100)
ht <- dnorm(z)
data <- data.frame(z=z, ht=ht)
zc <- 1.645
plot(data, type="n")
lines(data)
#segments(160,0,160,dnorm(160, m=160, sd=17), lty=2)
t <- subset(data, data$z>zc)
xvals <- t$z
dvals <- t$ht
polygon(c(xvals,rev(xvals)),c(rep(0,length(xvals)),rev(dvals)),col="red")

  ----- Original Message ----- 
  From: LL 
  To: r-help@stat.math.ethz.ch 
  Sent: Saturday, May 26, 2007 12:34 PM
  Subject: polygon error?


  Hi.. I'm not sure why polygon returns an area above the standard normal
curve.

  z <- pretty(c(-3,3), 100)
  ht <- dnorm(z)
  data <- data.frame(z=z, ht=ht)
  zc <- 1.645
  plot(data, type="l")
  lines(data)
  t <- subset(data, z>zc)
  polygon(t, col="red")

  Thanks,
  Lance


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