Created
May 21, 2012 16:23
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# Normal Distiribution | |
# Sequence between -4 and 4 with 0.1 steps | |
x <- seq(-4, 4, 0.1) | |
# Plot an empty chart with tight axis boundaries, and axis lines on bottom and left | |
plot(x, type="n", xaxs="i", yaxs="i", xlim=c(-4, 4), ylim=c(0, 0.4), | |
bty="l", xaxt="n", xlab="z-value", ylab="probability density") | |
# Function to plot each coloured portion of the curve, between "a" and "b" as a | |
# polygon; the function "dnorm" is the normal probability density function | |
polysection <- function(a, b, col, n=11){ | |
dx <- seq(a, b, length.out=n) | |
polygon(c(a, dx, b), c(0, dnorm(dx), 0), col=col, border=NA) | |
# draw a white vertical line on "inside" side to separate each section | |
segments(a, 0, a, dnorm(a), col="white") | |
} | |
# Build the four left and right portions of this bell curve | |
polysection( 0, 1, col="#2171B5") | |
polysection( 1, 1.96, col="#6BAED6") | |
polysection(-1, 0, col="#2171B5") | |
polysection(-1.96,-1, col="#6BAED6") | |
# Black outline of bell curve | |
lines(x, dnorm(x)) | |
# Bottom axis values, where sigma represents standard deviation and mu is the mean | |
axis(1, at=c(-3,-2,-1.96,-1,0,1,1.96,2,3), labels=expression(-3, '', -1.96, -1, 0, 1, 1.96, '', 3)) | |
# Add percent densities to each division, between x and x+1 | |
text( c(0,0), c(0.15,0.1), c('68%', '95%'), col=c("white","white")) |
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