predict.glm_weightit: Predictions for `glm_weightit` objects

Description

predict() generates predictions for models fit using glm_weightit(), ordinal_weightit(), multinom_weightit(), or coxph_weightit(). This page only details the predict() methods after using glm_weightit(), ordinal_weightit(), or multinom_weightit(). See survival::predict.coxph() for predictions when fitting Cox proportional hazards models using coxph_weightit().

Usage

# S3 method for glm_weightit
predict(object, newdata = NULL, type = "response", na.action = na.pass, ...)
# S3 method for ordinal_weightit
predict(
  object,
  newdata = NULL,
  type = "response",
  na.action = na.pass,
  values = NULL,
  ...
)
# S3 method for multinom_weightit
predict(
  object,
  newdata = NULL,
  type = "response",
  na.action = na.pass,
  values = NULL,
  ...
)

Value

A numeric vector containing the desired predictions, except for the following circumstances when an ordinal or multinomial model was fit:

when type = "response", a numeric matrix with a row for each unit and a column for each level of the outcome with the predicted probability of the corresponding outcome in the cells
when type = "class", a factor with the model predicted class for each unit; for ordinal models, this will be an ordered factor.

Arguments

object: a glm_weightit object.
newdata: optionally, a data frame in which to look for variables with which to predict. If omitted, the fitted values applied to the original dataset are used.
type: the type of prediction desired. Allowable options include "response", predictions on the scale of the original response variable (also "probs"); "link", predictions on the scale of the linear predictor (also "lp"); "class", the modal predicted category for ordinal and multinomial models; and "mean", the expected value of the outcome for ordinal and multinomial models. See Details for more information. The default is "response" for all models, which differs from stats::predict.glm().
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.
values: when type = "mean", the numeric values each level corresponds to. Should be supplied as a named vector with outcome levels as the names. If NULL and the outcome levels can be converted to numeric, those will be used. See Details.

Details

For generalized linear models other than ordinal and multinomial models, see stats::predict.glm() for more information on how predictions are computed and which arguments can be specified. Note that standard errors cannot be computed for the predictions using predict.glm_weightit().

For ordinal and multinomial models, setting type = "mean" computes the expected value of the outcome for each unit; this corresponds to the sum of the values supplied in values weighted by the predicted probability of those values. If values is omitted, predict() will attempt to convert the outcome levels to numeric values, and if this cannot be done, an error will be thrown. values should be specified as a named vector, e.g., values = c(one = 1, two = 2, three = 3), where "one", "two", and "three" are the original outcome levels and 1, 2, and 3 are the numeric values they correspond to. This method only makes sense to use if the outcome levels meaningfully correspond to numeric values.

For ordinal models, setting type = "link" (also "lp") computes the linear predictor without including the thresholds. This can be interpreted as the prediction of the latent variable underlying the ordinal response. This cannot be used with multinomial models.

Examples

Run this code

data("lalonde", package = "cobalt")

# Logistic regression model
fit1 <- glm_weightit(
  re78 > 0 ~ treat * (age + educ + race + married +
                        re74 + re75),
  data = lalonde, family = binomial, vcov = "none")

summary(predict(fit1))

# G-computation using predicted probabilities
p0 <- predict(fit1, type = "response",
              newdata = transform(lalonde,
                                  treat = 0))

p1 <- predict(fit1, type = "response",
              newdata = transform(lalonde,
                                  treat = 1))

mean(p1) - mean(p0)

# Multinomial logistic regression model
lalonde$re78_3 <- factor(findInterval(lalonde$re78,
                                      c(0, 5e3, 1e4)),
                         labels = c("low", "med", "high"))

fit2 <- multinom_weightit(
  re78_3 ~ treat * (age + educ + race + married +
                      re74 + re75),
  data = lalonde, vcov = "none")

# Predicted probabilities
head(predict(fit2))

# Class assignment accuracy
mean(predict(fit2, type = "class") == lalonde$re78_3)

# G-computation using expected value of the outcome
values <- c("low" = 2500,
            "med" = 7500,
            "high" = 12500)

p0 <- predict(fit2, type = "mean", values = values,
              newdata = transform(lalonde,
                                  treat = 0))

p1 <- predict(fit2, type = "mean", values = values,
              newdata = transform(lalonde,
                                  treat = 1))

mean(p1) - mean(p0)

# Ordinal logistic regression
fit3 <- ordinal_weightit(
  re78 ~ treat * (age + educ + race + married +
                    re74 + re75),
  data = lalonde, vcov = "none")

# G-computation using expected value of the outcome;
# using original outcome values
p0 <- predict(fit3, type = "mean",
              newdata = transform(lalonde,
                                  treat = 0))

p1 <- predict(fit3, type = "mean",
              newdata = transform(lalonde,
                                  treat = 1))

mean(p1) - mean(p0)