On Wed, 12 Aug 2009, Noah Silverman wrote:
Hi,
The answers to my previous question about nominal variables has lead me
to a more important question.
What is the "best practice" way to feed nominal variable to an SVM.
As some of the previous posters have already indicated: The data structure
for storing categorical (including nominal) variables in R is a "factor".
Your comment about "truly nominal" is wrong. A character variable is a
character variable, not necessarily a categorical variable. Categorical
means that the answer falls into one of a finite number of known
categories, known as "levels" in R's "factor" class.
If you start out from character information:
x <- c("red", "red", "blue", "green", "blue")
You can turn it into a factor via:
x <- factor(x, levels = c("red", "green", "blue"))
R now knows how to do certain things with such a variable, e.g., produces
useful summaries or knows how to deal with it in regression problems:
model.matrix(~ x)
which seems to be what you asked for. Moreover, you don't need call this
yourself but most regression functions in R will do that for you
(including svm() in "e1071" or ksvm() in "kernlab", among others).
In short: Keep your categorical variables as "factor" columns in a
"data.frame" and use the formula interface of svm()/ksvm() and you are
fine.
Z
For example:
color = ("red, "blue", "green")
I could translate that into an index so I wind up with
color= (1,2,3)
But my concern is that the SVM will now think that the values are numeric in
"range" and not discrete conditions.
Another thought would be to create 3 binary variables from the single color
variable, so I have:
red = (0,1)
blue = (0,1)
green = (0,1)
A example fed to the SVM would have one positive and two negative values to
indicate the color value:
i.e. for a blue example:
red = 0, blue =1 , green = 0
Or, do any of the SVM packages intelligently handle this internally so that I
don't have to mess with it. If so, do I need to be concerned about different
"translation" of the data if the test data set isn't exactly the same as the
training set.
For example:
training data = color ("red, "blue", "green")
test data = color ("red, "green")
How would I be sure that the "red" and "green" examples get encoded the same
so that the SVM is accurate?
Thanks in advance!!
-N
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