Technical Details: Adding own class-methods to ggeffects

get_predictions() is the core function to return adjusted predictions for a model, when calling ggpredict() or predict_response() with margin = "mean_reference" (the default option for margin). Basically, the input contains the model object and a data grid that is typically used for the newdata argument of the predict() method. get_predictions() can be used as S3-method for own classes, to add support for new models in ggeffects and is only relevant for package developers.

There are no S3-class definitions for ggemmeans() or ggaverage(), because these functions simply call methods from the emmeans or marginaleffects packages. Hence, methods should be written for those packages, too, if a model-object should work with ggemmeans() or ggaverage().

Input: Arguments that are passed to get_predictions()

Adding support for ggpredict() (or: predict_response() with default options) is quite easy. In order to make your model class work with ggpredict(), you need to add a get_predictions() method. get_predictions() is called from predict_response() and following arguments are always passed to get_predictions() - no matter if the corresponding class requires all of those arguments or not:

• model, data_grid, terms, ci_level, model_info, type, typical, vcov, vcov_args, condition, interval, link_inverse, bias_correction, verbose

Please refer to the documentation of predict_response() for explanation of these arguments

It is not necessary to process all of those arguments, but they can be used to modulate certain settings when calculating predictions. Note that if your method does not define all mentioned arguments, these are still passed via ... - make sure that further methods in your get_predictions() method still work when they process the ....

Output: the data.frame structure

It is important that the function returns a data frame with a specific structure, namely the data grid (argument data_grid, that was passed to get_predictions()), including the additional columns predicted, conf.low, and conf.high (which are the results of the get_predictions() function). Note that predictions and intervals usually should be on the response scale.

Example

A simple example for an own class-implementation for Gaussian-alike models could look like this:

get_predictions.own_class <- function(model, data_grid, ci_level = 0.95, ...) {
  predictions <- predict(
    model,
    newdata = data_grid,
    type = "response",
    se.fit = !is.na(ci_level),
    ...
  )

  # do we have standard errors?
  if (is.na(ci_level)) {
    # copy predictions
    data_grid$predicted <- as.vector(predictions)
  } else {
    # copy predictions
    data_grid$predicted <- predictions$fit

    # calculate CI
    data_grid$conf.low <- predictions$fit - qnorm(0.975) * predictions$se.fit
    data_grid$conf.high <- predictions$fit + qnorm(0.975) * predictions$se.fit

    # optional: copy standard errors
    attr(data_grid, "std.error") <- predictions$se.fit
  }

  data_grid
}

A simple example for an own class-implementation for non-Gaussian-alike models could look like this (note the use of the link-inverse function link_inverse(), which is passed to the link_inverse argument):

get_predictions.own_class <- function(model,
                                      data_grid,
                                      ci_level = 0.95,
                                      link_inverse = insight::link_inverse(model),
                                      ...) {
  predictions <- predict(
    model,
    newdata = data_grid,
    type = "link", # for non-Gaussian, return on link-scale
    se.fit = !is.na(ci_level),
    ...
  )

  # do we have standard errors?
  if (is.na(ci_level)) {
    # copy predictions
    data_grid$predicted <- link_inverse(as.vector(predictions))
  } else {
    # copy predictions, use link-inverse to back-transform
    data_grid$predicted <- link_inverse(predictions$fit)

    # calculate CI
    data_grid$conf.low <- link_inverse(
      predictions$fit - qnorm(0.975) * predictions$se.fit
    )
    data_grid$conf.high <- link_inverse(
      predictions$fit + qnorm(0.975) * predictions$se.fit
    )

    # optional: copy standard errors
    attr(data_grid, "std.error") <- predictions$se.fit
  }

  data_grid
}