Calculates the change in predicted counts or optionally the predicted probability of being in the zero-count group, for maximal discrete changes in all covariates holding all other variables constant at typical values.
ziChange(obj, data, typical.dat = NULL, type = "count")A model object of class zeroinfl.
Data frame used to fit object.
Data frame with a single row containing values at which
to hold variables constant when calculating first differences. These values
will be passed to predict, so factors must take on a single value,
but have all possible levels as their levels attribute.
Character string of either ‘count’ (to obtain changes in predicted counts) or ‘zero’ (to obtain changes in the predicted probability of membership in the zero group).
A list with the following elements:
A matrix of calculated first differences
A matrix of values that were used to calculate the predicted changes
The function calculates the changes in predicted counts, or optionally the
predicted probability of being in the zero group, for maximal discrete
changes in the covariates. This function works with polynomials specified
with the poly function. It also works with multiplicative
interactions of the covariates by virtue of the fact that it holds all other
variables at typical values. By default, typical values are the median for
quantitative variables and the mode for factors. The way the function works
with factors is a bit different. The function identifies the two most
different levels of the factor and calculates the change in predictions for
a change from the level with the smallest prediction to the level with the
largest prediction.