Code for Figure 2.5 on page 30 of Statistical Rethinking

figure2_5_page30.R

## code to reproduce Figure 2.5 on page 30 of Statistical Rethinking

library(rethinking)

col1 <- "black"
col2 <- col.alpha( "black" , 0.7 )

# show posterior as data comes in
# 1 indicates 'water'; 0 indicates 'land'
d <- c(1,0,1,1,1,0,1,0,1) # length 9
l <- ifelse( d==1 , "W" , "L" )

par(mfrow=c(3,3)) # 3 by 3 grid
# make nicer margins
par(mgp = c(1.5, 0.5, 0), mar = c(2.5, 2.5, 2, 1) + 0.1, tck = -0.02 )

# prior uniform - "Laplace" prior
a <- 1
b <- 1

# draw plots
for ( i in 1:9 ) {
    prior.a <- a
    prior.b <- b
    a <- a + d[i]
    b <- b + 1 - d[i]
    curve( dbeta(x,a,b) , from=0 , to=1 , xlab="probability of water" , ylab="" , col=col1 , lwd=2 , ylim=c(0,2.75) , xaxt="n" , yaxt="n" )
    axis( 1 , at=c(0,0.5,1) , labels=c(0,0.5,1) )
    text( 0.15 , 2.5 , paste("n =",i) )
    curve( dbeta(x,prior.a,prior.b) , add=TRUE , col=col1 , lty=2 )
    mtext( paste(l) , at=seq(from=0,to=1,length.out=length(l)) , col=c( rep(col1,i) , rep( "darkgray" ,length(l)-i) ) )
}