[R] How to plot a normal distribution curve and a shaded tail with alpha?

[Joshua Wiley]

I wrote this a bit ago, its far from perfect, but might give you
ideas/serve your purpose.

HTH,

Josh

plot.dist <- function(alpha, from = -5, to = 5, n = 1000, filename = NULL,
alternative = c("two.tailed", "greater", "lesser"),
distribution = c("normal", "t", "F", "chisq", "binomial"),
colour = "black", fill = "skyblue2", ...) {
alternative <- match.arg(alternative)
## Calculate alpha level given hypothesis
alt.alpha <- switch(alternative,
two.tailed = alpha/2,
greater = alpha,
lesser = alpha)
## use a ’switch’ to pick the right functions based on distribution
my.den <- switch(distribution,
normal = dnorm,
t = dt,
F = df,
chisq = dchisq,
binomial = dbinom)
my.dist <- switch(distribution,
normal = qnorm,
t = qt,
F = qf,
chisq = qchisq,
binomial = qbinom)
## Additional arguments passed via ’...’ e.g., degrees of freedom
crit.lower <- my.dist(p = alt.alpha, lower.tail = TRUE, ...)
crit.upper <- my.dist(p = alt.alpha, lower.tail = FALSE, ...)
## Calculate alpha (lower) region coordinates
cord.x1 <- c(from, seq(from = from, to = crit.lower,
length.out = 100), crit.lower)
cord.y1 <- c(0, my.den(x = seq(from = from, to = crit.lower,
length.out = 100), ...), 0)
## Calculate alpha (upper) region coordinates
cord.x2 <- c(crit.upper, seq(from = crit.upper, to = to,
length.out = 100), to)
cord.y2 <- c(0, my.den(x = seq(from = crit.upper, to = to,
length.out = 100), ...), 0)
## Logic test to choose which graphic device to open
if(is.null(filename)) {
dev.new()
} else {
pdf(file = filename)
}
## plot distribution
curve(my.den(x, ...), from = from, to = to,
n = n, col = colour, lty = 1, lwd = 2,
ylab = "Density", xlab = "Values")
## Add alpha region(s) based on given hypothesis
if(!identical(alternative, "greater")) {
polygon(x = cord.x1, y = cord.y1, col = fill)
}
if(!identical(alternative, "lesser")) {
polygon(x = cord.x2, y = cord.y2, col = fill)
}
## If the PDF device was started, shut it down
if(!is.null(filename)) {dev.off()}
}


On Mon, Nov 8, 2010 at 8:18 AM, Wu Gong <wg2f at mtmail.mtsu.edu> wrote:
>
> I want to create a graph to express the idea of the area under a pdf curve,
> like
>
> http://r.789695.n4.nabble.com/file/n3032194/w7295e04.jpg
>
> Thank you for any help.
>
> -----
> A R learner.
> --
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> Sent from the R help mailing list archive at Nabble.com.
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/