CODEWITHSUNDEEP
rplot
Jonas Roberts
Jonas Roberts

Reputation: 33

Difference between two density plots

Is there a simple way to plot the difference between two probability density functions?

I can plot the pdfs of my data sets (both are one-dimensional vectors with roughly 11000 values) on the same plot together to get an idea of the overlap/difference but it would be more useful to me if I could see a plot of the difference.

something along the lines of the following (though this obviously doesn't work):

> plot(density(data1)-density(data2))

I'm relatively new to R and have been unable to find what I'm looking for on any of the forums.

Thanks in advance

Upvotes: 3

Views: 5121

Answers (2)

Carl Witthoft
Carl Witthoft

Reputation: 21502

It looks like you need to spend a little time learning how to use R (or any other language, for that matter). Help files are your friend. From the output of ?density :

Value [i.e. the data returned by the function]

If give.Rkern is true, the number R(K), otherwise an object with class "density" whose underlying structure is a list containing the following components.

x the n coordinates of the points where the density is estimated.

y the estimated density values. These will be non-negative, but can be zero [remainder of "value" deleted for brevity]

So, do: 

foo<- density(data1) 
bar<- density(data2)
plot(foo$y-bar$y)

Upvotes: 1

IRTFM
IRTFM

Reputation: 263362

This should work:

plot(x =density(data1, from= range(c(data1, data2))[1], 
                       to=range(c(data1, data2))[2] )$x, 
  y=  density(data1, from= range(c(data1, data2))[1], 
                     to=range(c(data1, data2))[2] )$y-
       density(data2,  from= range(c(data1, data2))[1], 
                      to=range(c(data1, data2))[2] )$y )

The trick is to make sure the densities have the same limits. Then you can plot their differences at the same locations.My understanding of the need for the identical limits comes from having made the error of not taking that step in answering a similar question on Rhelp several years ago. Too bad I couldn't remember the right arguments.

Upvotes: 5

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