keyness {DramaAnalysis}

Keywords

Description

Given a frequency table (with texts as rows and words as columns), this function calculates log-likelihood and log ratio of one set of rows against the other rows. The return value is a list containing scores for each word. If the method is loglikelihood, the returned scores are unsigned G2 values. To estimate the direction of the keyness, the log ratio is more informative. A nice introduction into log ratio can be found here.

Usage

keyness(
  ft,
  categories = c(1, rep(2, nrow(ft) - 1)),
  epsilon = 1e-100,
  siglevel = 0.05,
  method = c("loglikelihood", "logratio"),
  minimalFrequency = 10
)

Arguments

ft

The frequency table

categories

A factor or numeric vector that represents an assignment of categories.

epsilon

null values are replaced by this value, in order to avoid division by zero

siglevel

Return only the keywords above the significance level. Set to 1 to get all words

method

Either "logratio" or "loglikelihood" (default)

minimalFrequency

Words less frequent than this value are not considered at all

Value

A list of keywords, sorted by their log-likelihood or log ratio value, calculated according to http://ucrel.lancs.ac.uk/llwizard.html.

Examples

data("rksp.0")
ft <- frequencytable(rksp.0, byCharacter = TRUE, normalize = FALSE)
# Calculate log ratio for all words
genders <- factor(c("m", "m", "m", "m", "f", "m", "m", "m", "f", "m", "m", "f", "m"))
keywords <- keyness(ft, method = "logratio", 
                    categories = genders, 
                    minimalFrequency = 5)
# Remove words that are not significantly different
keywords <- keywords[names(keywords) %in% names(keyness(ft, siglevel = 0.01))]