R: Alligator Food Choice

Alligator {vcdExtra} | R Documentation

## Alligator Food Choice

### Description

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).

### Usage

    data(Alligator)

### Format

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

### Details

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.

### Source

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

### Examples

    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)
    }

[Package vcdExtra version 0.7-1 Index]