Benford’s law, or the First-digit law

August 25, 2011
By

(This article was first published on Statistic on aiR, and kindly contributed to R-bloggers)

Benford’s law, also called the first-digit law, states that in lists of numbers from many (but not all) real-life sources of data, the leading digit is distributed in a specific, non-uniform way. According to this law, the first digit is 1 about 30% of the time, and larger digits occur as the leading digit with lower and lower frequency, to the point where 9 as a first digit occurs less than 5% of the time.

Wikipedia, retrieved 08/25/2011

R simulation:

``` library(MASS) benford <- function(m, n){ list <- c() # compute all m^n, for n= 1, 2, ..., i, ..., n for(i in 1:n){ list[i] <- m^i }```

``` # a function to extract the first digit from a number bben <- function(k){ as.numeric(head(strsplit(as.character(k),'')[[1]],n=1)) } # extract the first digit from all numbers computed first.digit <- sapply(list, bben) # plot frequency of first digits truehist(first.digit, nbins=10, main=m) } ```

```par(mfrow=c(2,2)) benford(2,1000) benford(3,640) # if n is greater, it returns "inf" (on my pc) benford(4,500) benford(5,440) ```

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