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#' @title Satterthwaite approximation for SEs, CIs and p-values
#' @name p_value_satterthwaite
#'
#' @description An approximate F-test based on the Satterthwaite (1946) approach.
#'
#' @param model A statistical model.
#' @param dof Degrees of Freedom.
#' @inheritParams ci.default
#'
#' @details Inferential statistics (like p-values, confidence intervals and
#' standard errors) may be biased in mixed models when the number of clusters
#' is small (even if the sample size of level-1 units is high). In such cases
#' it is recommended to approximate a more accurate number of degrees of freedom
#' for such inferential statistics. Unlike simpler approximation heuristics
#' like the "m-l-1" rule (`dof_ml1`), the Satterthwaite approximation is
#' also applicable in more complex multilevel designs. However, the "m-l-1"
#' heuristic also applies to generalized mixed models, while approaches like
#' Kenward-Roger or Satterthwaite are limited to linear mixed models only.
#'
#' @seealso `dof_satterthwaite()` and `se_satterthwaite()` are small helper-functions
#' to calculate approximated degrees of freedom and standard errors for model
#' parameters, based on the Satterthwaite (1946) approach.
#'
#' [`dof_kenward()`] and [`dof_ml1()`] approximate degrees of freedom based on
#' Kenward-Roger's method or the "m-l-1" rule.
#'
#' @examples
#' \donttest{
#' if (require("lme4", quietly = TRUE)) {
#' model <- lmer(Petal.Length ~ Sepal.Length + (1 | Species), data = iris)
#' p_value_satterthwaite(model)
#' }
#' }
#' @return A data frame.
#' @references Satterthwaite FE (1946) An approximate distribution of estimates of variance components. Biometrics Bulletin 2 (6):110–4.
#' @export
p_value_satterthwaite <- function(model, dof = NULL, ...) {
if (is.null(dof)) {
dof <- dof_satterthwaite(model)
}
.p_value_dof(model, dof, method = "satterthwaite", ...)
}
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