[R] Understanding R Hist() Results...

[(Ted Harding)]

On 04-Jun-09 04:00:11, Jason Rupert wrote:
> 
> Think I'm missing something to understand what is going on with
> hist(...)
> 
> http://n2.nabble.com/What-is-going-on-with-Histogram-Plots-td3022645.htm
> l
> 
> For my example I count 7 unique years, however, on the histogram there
> only 6.  It looks like the bin to the left of the tic mark on the
> x-axis represents the number of entries for that year, i.e. Frequency. 
> 
> I guess it looks like the bin for 1990 is missing.  Is there a better
> way or a different histogram R command to use in order to see all the
> age bins and them for them to be aligned directly over the year tic
> mark on the x-axis?  
> 
> Thanks again for any insights that can be provided.

It's doing what it's supposed to -- which admitredly could be confusing
when all your data lie on the exact boundaries between bins.

>From ?hist, by default "include.lowest = TRUE, right = TRUE", and:

  If 'right = TRUE' (default), the histogram cells are intervals of
  the form '(a, b]', i.e., they include their right-hand endpoint,
  but not their left one, with the exception of the first cell when
  'include.lowest' is 'TRUE'.

In your data:

 sort(HouseYear_array)
 [1] "1990" "1991" "1992" "1993" "1993" "1993" "1993" "1994" "1994"
[10] "1994" "1994" "1995" "1995" "1995" "1995" "1995" "1995" "1995"
[20] "1995" "1996" "1996" "1996" "1996" "1996" "1996" "1996" "1996"

and, with

  H<-hist(as.numeric(HouseYear_array))
  H$breaks
  # [1] 1990 1991 1992 1993 1994 1995 1996

so you get 2 (1990,1991) in the [1990-1] bin, 1 in the [1991-2] bin,
4 in [1992-3], and so on, exactly as observed.

You can get what you're expecting to see by setting the 'breaks'
parameter explicitly, and making sure the breakpoints do not
coincide with data (which ensures that there is no confusion about
what goes in which bin):

  hist(as.numeric(HouseYear_array),breaks=0.5+(1989:1996))

Ted.

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Date: 04-Jun-09                                       Time: 11:13:22
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