[Rd] Unexpected behavior in factor level ordering

Paul Johnson pauljohn32 at gmail.com
Sat Feb 25 19:16:07 CET 2012

Hello, Everybody:

This may not be a "bug", but for me it is an unexpected outcome. A
factor variable's levels
do not retain their ordering after the levels function is used.  I
supply an example in which
a factor with values "BC" "AD" (in that order) is unintentionally
re-alphabetized by the levels

To me, this is very bad behavior. Would you agree?

# Paul Johnson 2012-02-05

x <- c("AD","BC","AD","BC","AD","BC")
xf <- factor(x, levels=c("BC", "AD"), labels=c("Before Christ","After Christ"))
y <- rnorm(6)

m1 <- lm (y ~ xf )

plot(y ~ xf)

abline (m1)
## Just a little problem the line does not "go through" the box
## plot in the right spot because contrasts(xf) is 0,1 but
## the plot uses xf in 1,2.

xlevels <- levels(xf)
newdf <- data.frame(xf=xlevels)

ypred <- predict(m1, newdata=newdf)

##Watch now: the plot comes out "reversed", AC before BC
plot(ypred ~ newdf$xf)

## Ah. Now I see:

## Why doesnt newdf$xf respect the ordering of the levels?

Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas

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