BradleyTerry2 (version 1.1-2)

predict.BTm: Predict Method for Bradley-Terry Models

Description

Obtain predictions and optionally standard errors of those predictions from a fitted Bradley-Terry model.

Usage

# S3 method for BTm
predict(
  object,
  newdata = NULL,
  level = ifelse(is.null(object$random), 0, 1),
  type = c("link", "response", "terms"),
  se.fit = FALSE,
  dispersion = NULL,
  terms = NULL,
  na.action = na.pass,
  ...
)

Value

If se.fit = FALSE, a vector or matrix of predictions. If se = TRUE, a list with components

fit

Predictions

se.fit

Estimated standard errors

Arguments

object

a fitted object of class "BTm"

newdata

(optional) a data frame in which to look for variables with which to predict. If omitted, the fitted linear predictors are used.

level

for models with random effects: an integer vector giving the level(s) at which predictions are required. Level zero corresponds to population-level predictions (fixed effects only), whilst level one corresponds to the player-level predictions (full model) which are NA for contests involving players not in the original data. By default, level = 0 for a fixed effects model, 1 otherwise.

type

the type of prediction required. The default is on the scale of the linear predictors; the alternative "response" is on the scale of the response variable. Thus for a default Bradley-Terry model the default predictions are of log-odds (probabilities on logit scale) and type = "response" gives the predicted probabilities. The "terms" option returns a matrix giving the fitted values of each term in the model formula on the linear predictor scale (fixed effects only).

se.fit

logical switch indicating if standard errors are required.

dispersion

a value for the dispersion, not used for models with random effects. If omitted, that returned by summary applied to the object is used, where applicable.

terms

with type ="terms" by default all terms are returned. A character vector specifies which terms are to be returned.

na.action

function determining what should be done with missing values in newdata. The default is to predict NA.

...

further arguments passed to or from other methods.

Author

Heather Turner

Details

If newdata is omitted the predictions are based on the data used for the fit. In that case how cases with missing values in the original fit are treated is determined by the na.action argument of that fit. If na.action = na.omit omitted cases will not appear in the residuals, whereas if na.action = na.exclude they will appear (in predictions and standard errors), with residual value NA. See also napredict.

See Also

predict.glm(), predict.glmmPQL()

Examples

## The final model in example(flatlizards)
result <- rep(1, nrow(flatlizards$contests))
Whiting.model3 <- BTm(1, winner, loser, ~ throat.PC1[..] + throat.PC3[..] +
                      head.length[..] + SVL[..] + (1|..),
                      family = binomial(link = "probit"),
                      data = flatlizards, trace = TRUE)

## `new' data for contests between four of the original lizards
## factor levels must correspond to original levels, but unused levels
## can be dropped - levels must match rows of predictors
newdata  <- list(contests = data.frame(
                 winner = factor(c("lizard048", "lizard060"),
                 levels = c("lizard006", "lizard011", 
                            "lizard048", "lizard060")),
                 loser = factor(c("lizard006", "lizard011"),
                 levels = c("lizard006", "lizard011", 
                            "lizard048", "lizard060"))
                 ),
                 predictors = flatlizards$predictors[c(3, 6, 27, 33), ])

predict(Whiting.model3, level = 1, newdata = newdata)

## same as
predict(Whiting.model3, level = 1)[1:2]

## introducing a new lizard
newpred <- rbind(flatlizards$predictors[c(3, 6, 27),
                     c("throat.PC1","throat.PC3", "SVL", "head.length")],
                 c(-5, 1.5, 1, 0.1))
rownames(newpred)[4] <- "lizard059"

newdata  <- list(contests = data.frame(
                 winner = factor(c("lizard048", "lizard059"),
                 levels = c("lizard006", "lizard011", 
                            "lizard048", "lizard059")),
                 loser = factor(c("lizard006", "lizard011"),
                 levels = c("lizard006", "lizard011", 
                            "lizard048", "lizard059"))
                 ),
                 predictors = newpred)

## can only predict at population level for contest with new lizard
predict(Whiting.model3, level = 0:1, se.fit = TRUE, newdata = newdata)

## predicting at specific levels of covariates

## consider a model from example(CEMS)
table6.model <-  BTm(outcome = cbind(win1.adj, win2.adj),
                     player1 = school1, player2 = school2,
                     formula = ~ .. +
                         WOR[student] * Paris[..] +
                         WOR[student] * Milano[..] +
                         WOR[student] * Barcelona[..] +
                         DEG[student] * St.Gallen[..] +
                         STUD[student] * Paris[..] +
                         STUD[student] * St.Gallen[..] +
                         ENG[student] * St.Gallen[..] +
                         FRA[student] * London[..] +
                         FRA[student] * Paris[..] +
                         SPA[student] * Barcelona[..] +
                         ITA[student] * London[..] +
                         ITA[student] * Milano[..] +
                         SEX[student] * Milano[..],
                     refcat = "Stockholm",
                     data = CEMS)
                     
## estimate abilities for a combination not seen in the original data

## same schools
schools <- levels(CEMS$preferences$school1)
## new student data
students <- data.frame(STUD = "other", ENG = "good", FRA = "good", 
                       SPA = "good", ITA = "good", WOR = "yes", DEG = "no",
                       SEX = "female", stringsAsFactors = FALSE)
## set levels to be the same as original data    
for (i in seq_len(ncol(students))){
    students[,i] <- factor(students[,i], levels(CEMS$students[,i]))
}
newdata <- list(preferences = 
    data.frame(student = factor(500), # new id matching with `students[1,]`
               school1 = factor("London", levels = schools),
               school2 = factor("Paris", levels = schools)),
    students = students,
    schools = CEMS$schools)

## warning can be ignored as model specification was over-parameterized
predict(table6.model, newdata = newdata)

## if treatment contrasts are use (i.e. one player is set as the reference
## category), then predicting the outcome of contests against the reference
## is equivalent to estimating abilities with specific covariate values

## add student with all values at reference levels 
students <- rbind(students,
    data.frame(STUD = "other", ENG = "good", FRA = "good", 
               SPA = "good", ITA = "good", WOR = "no", DEG = "no",
               SEX = "female", stringsAsFactors = FALSE))
## set levels to be the same as original data    
for (i in seq_len(ncol(students))){
    students[,i] <- factor(students[,i], levels(CEMS$students[,i]))
}
newdata <- list(preferences = 
    data.frame(student = factor(rep(c(500, 502), each = 6)), 
               school1 = factor(schools, levels = schools),
               school2 = factor("Stockholm", levels = schools)),
    students = students,
    schools = CEMS$schools)
    
predict(table6.model, newdata = newdata, se.fit = TRUE)

## the second set of predictions (elements 7-12) are equivalent to the output 
## of BTabilities; the first set are adjust for `WOR` being equal to "yes"
BTabilities(table6.model)