Archive for perplexity

Typos in Introduction to Monte Carlo Methods with R

Posted in Books, R, Statistics, University life with tags , , , , on October 13, 2011 by xi'an

The two translators of our book in Japanese, Kazue & Motohiro Ishida, contacted me about some R code mistakes in the book. The translation is nearly done and they checked every piece of code in the book, an endeavour for which I am very grateful! Here are the two issues they have noticed (after incorporating the typos signaled in the overall up-to-date summary):

First, in Example 4.4, I omitted some checkings and forgot about a minus sign, meaning Figure 4.4 (right) is wrong. (The more frustrating since this example covers perplexity!) The zeros must be controlled via code lines like

> wachd[wachd<10^(-10)]=10^(-10)

instead of the meaningless

wachd[apply(wachd,2,cumsum)<10^(-10)]=10^(-10)

and the addition of

> plex[plex>0]=0
> plech[plech>0]=0

after the definition of those two variables.  (Because entropies are necessarily positive.) The most glaring omission is however the minus in

> plob=apply(exp(-plex),1,quantile,c(.025,.975))
> ploch=apply(exp(-plech),1,quantile,c(.025,.975))

which modifies Figure 4.4 in the following

The second case is Example 7.3 where I forgot to account for the log-transform of the data, which should read (p.204):

> x=c(91,504,557,609,693,727,764,803,857,929,970,1043,
+     1089,1195,1384,1713)
> x=log(x)

and compounded my mistake by including log-transforms of the parameters that should not be there (pp.204-205)! So (for my simulations) the posterior means of θ and σ² are 6.62 and 0.661, respectively, leading to an estimate of σ of 0.802. There should be no log transform in Exercise 7.3 either.

The same corrections apply to the French translation, most obviously…

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