Alligator Food Choice

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)
#>                       food bird fish invert other reptile
#> lake     sex    size                                     
#> George   female large         0    8      1     1       0
#>                 small         0    3      9     1       1
#>          male   large         1    9      0     2       0
#>                 small         2   13     10     2       0
#> Hancock  female large         2    3      0     3       1
#>                 small         2   16      3     3       2
#>          male   large         1    4      0     2       0
#>                 small         0    7      1     5       0
#> Oklawaha female large         1    0      1     0       0
#>                 small         0    3      9     2       1
#>          male   large         0   13      7     0       6
#>                 small         0    2      2     1       0
#> Trafford female large         0    0      1     0       0
#>                 small         1    2      4     4       1
#>          male   large         3    8      6     5       6
#>                 small         0    3      7     1       1


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)
}
#> Loading required package: nnet
#> # weights:  35 (24 variable)
#> initial  value 352.466903 
#> iter  10 value 270.397070
#> iter  20 value 268.958046
#> final  value 268.932740 
#> converged