Alligator: Alligator Food Choice
In vcdExtra: 'vcd' Extensions and Additions

Description Usage Format Details Source Examples

The Alligator data, from Agresti (2002), comes from a study of the primary food choices of alligators in four Florida lakes. Researchers classified the stomach contents of 219 captured alligators into five categories: Fish (the most common primary food choice), Invertebrate (snails, insects, crayfish, etc.), Reptile (turtles, alligators), Bird, and Other (amphibians, plants, household pets, stones, and other debris).

1	data(Alligator)

A frequency data frame with 80 observations on the following 5 variables.

lake: a factor with levels George Hancock Oklawaha Trafford
sex: a factor with levels female male
size: alligator size, a factor with levels large (>2.3m) small (<=2.3m)
food: primary food choice, a factor with levels bird fish invert other reptile
count: cell frequency, a numeric vector

The table contains a fair number of 0 counts.

food is the response variable. fish is the most frequent choice, and often taken as a baseline category in multinomial response models.

Agresti, A. (2002). Categorical Data Analysis, New York: Wiley, 2nd Ed., Table 7.1

data(Alligator)

# change from frequency data.frame to table
allitable <- xtabs(count~lake+sex+size+food, data=Alligator)
# Agresti's Table 7.1
structable(food~lake+sex+size, allitable)


plot(allitable, shade=TRUE)
# mutual independence model
mosaic(~food+lake+size, allitable, shade=TRUE)
# food jointly independent of lake and size
mosaic(~food+lake+size, allitable, shade=TRUE, expected=~lake:size+food)

if (require(nnet)) {
	# multinomial logit model
	mod1 <- multinom(food ~ lake+size+sex, data=Alligator, weights=count)
}