more accuracy for p_sig/p_dir

DESCRIPTION

@@ -113,6 +113,7 @@ Suggests:
     coxme,
     cplm,
     dbscan,
+    distributional,
     domir (>= 0.2.0),
     drc,
     DRR,

R/equivalence_test.R

@@ -10,15 +10,14 @@ bayestestR::equivalence_test
 #'
 #' @param x A statistical model.
 #' @param range The range of practical equivalence of an effect. May be
-#'   `"default"`, to automatically define this range based on properties of the
-#'   model's data.
+#' `"default"`, to automatically define this range based on properties of the
+#' model's data.
 #' @param ci Confidence Interval (CI) level. Default to `0.95` (`95%`).
 #' @param rule Character, indicating the rules when testing for practical
-#'   equivalence. Can be `"bayes"`, `"classic"` or `"cet"`. See
-#'   'Details'.
+#' equivalence. Can be `"bayes"`, `"classic"` or `"cet"`. See 'Details'.
 #' @param test Hypothesis test for computing contrasts or pairwise comparisons.
-#'   See [`?ggeffects::test_predictions`](https://strengejacke.github.io/ggeffects/reference/test_predictions.html)
-#'   for details.
+#' See [`?ggeffects::test_predictions`](https://strengejacke.github.io/ggeffects/reference/test_predictions.html)
+#' for details.
 #' @param verbose Toggle warnings and messages.
 #' @param ... Arguments passed to or from other methods.
 #' @inheritParams model_parameters.merMod
@@ -28,18 +27,17 @@ bayestestR::equivalence_test
 #' readings can be found in the references. See also [`p_significance()`] for
 #' a unidirectional equivalence test.
 #'
-#' @details
-#' In classical null hypothesis significance testing (NHST) within a frequentist
-#' framework, it is not possible to accept the null hypothesis, H0 - unlike
-#' in Bayesian statistics, where such probability statements are possible.
-#' "[...] one can only reject the null hypothesis if the test
+#' @details In classical null hypothesis significance testing (NHST) within a
+#' frequentist framework, it is not possible to accept the null hypothesis, H0 -
+#' unlike in Bayesian statistics, where such probability statements are
+#' possible. "[...] one can only reject the null hypothesis if the test
 #' statistics falls into the critical region(s), or fail to reject this
 #' hypothesis. In the latter case, all we can say is that no significant effect
 #' was observed, but one cannot conclude that the null hypothesis is true."
-#' (_Pernet 2017_). One way to address this issues without Bayesian methods
-#' is *Equivalence Testing*, as implemented in `equivalence_test()`.
-#' While you either can reject the null hypothesis or claim an inconclusive result
-#' in NHST, the equivalence test - according to _Pernet_ - adds a third category,
+#' (_Pernet 2017_). One way to address this issues without Bayesian methods is
+#' *Equivalence Testing*, as implemented in `equivalence_test()`. While you
+#' either can reject the null hypothesis or claim an inconclusive result in
+#' NHST, the equivalence test - according to _Pernet_ - adds a third category,
 #' *"accept"*. Roughly speaking, the idea behind equivalence testing in a
 #' frequentist framework is to check whether an estimate and its uncertainty
 #' (i.e. confidence interval) falls within a region of "practical equivalence".
@@ -95,25 +93,25 @@ bayestestR::equivalence_test
 #' ## p-Values
 #' The equivalence p-value is the area of the (cumulative) confidence
 #' distribution that is outside of the region of equivalence. It can be
-#' interpreted as p-value for *rejecting* the alternative hypothesis
-#' and *accepting* the "null hypothesis" (i.e. assuming practical
-#' equivalence). That is, a high p-value means we reject the assumption of
-#' practical equivalence and accept the alternative hypothesis.
+#' interpreted as p-value for *rejecting* the alternative hypothesis and
+#' *accepting* the "null hypothesis" (i.e. assuming practical equivalence). That
+#' is, a high p-value means we reject the assumption of practical equivalence
+#' and accept the alternative hypothesis.
 #'
 #' ## Second Generation p-Value (SGPV)
 #' Second generation p-values (SGPV) were proposed as a statistic that
 #' represents _the proportion of data-supported hypotheses that are also null
 #' hypotheses_ _(Blume et al. 2018, Lakens and Delacre 2020)_. It represents the
-#' proportion of the _full_ confidence interval range (assuming a normally
-#' distributed, equal-tailed interval) that is inside the ROPE. The SGPV ranges
-#' from zero to one. Higher values indicate that the effect is more likely to be
-#' practically equivalent ("not of interest").
+#' proportion of the _full_ confidence interval range (assuming a normally or
+#' t-distributed, equal-tailed interval, based on the model) that is inside the
+#' ROPE. The SGPV ranges from zero to one. Higher values indicate that the
+#' effect is more likely to be practically equivalent ("not of interest").
 #'
 #' Note that the assumed interval, which is used to calculate the SGPV, is an
 #' estimation of the _full interval_ based on the chosen confidence level. For
 #' example, if the 95% confidence interval of a coefficient ranges from -1 to 1,
-#' the underlying _full (normally distributed) interval_ approximately ranges
-#' from -1.9 to 1.9, see also following code:
+#' the underlying _full (normally or t-distributed) interval_ approximately
+#' ranges from -1.9 to 1.9, see also following code:
 #'
 #' ```
 #' # simulate full normal distribution
@@ -390,6 +388,7 @@ equivalence_test.ggeffects <- function(x,
   focal <- attributes(x)$original.terms
   obj_name <- attributes(x)$model.name
   ci <- attributes(x)$ci.lvl
+  dof <- attributes(x)$df
 
   x <- .get_ggeffects_model(x)
 
@@ -419,6 +418,7 @@ equivalence_test.ggeffects <- function(x,
         ci_narrow,
         range_rope = range,
         rule = rule,
+        dof = dof,
         verbose = verbose
       )
     }, conf_int, conf_int2
@@ -490,6 +490,18 @@ equivalence_test.ggeffects <- function(x,
   }
 
 
+  # ==== check degrees of freedom ====
+
+  df_column <- grep("(df|df_error)", colnames(x))
+  if (length(df_column) > 0) {
+    dof <- unique(x[[df_column]])
+    if (length(dof) > 1) {
+      dof <- Inf
+    }
+  } else {
+    dof <- Inf
+  }
+
   # ==== requested confidence intervals ====
 
   params <- conf_int <- .ci_generic(x, ci = ci)
@@ -513,6 +525,7 @@ equivalence_test.ggeffects <- function(x,
         ci_narrow,
         range_rope = range,
         rule = rule,
+        dof = dof,
         verbose = verbose
       )
     }, conf_int, conf_int2
@@ -623,6 +636,7 @@ equivalence_test.ggeffects <- function(x,
                                       ci_narrow,
                                       range_rope,
                                       rule,
+                                      dof = Inf,
                                       verbose) {
   final_ci <- NULL
 
@@ -674,7 +688,7 @@ equivalence_test.ggeffects <- function(x,
   data.frame(
     CI_low = final_ci[1],
     CI_high = final_ci[2],
-    SGPV = .rope_coverage(ci = ci, range_rope, ci_range = final_ci),
+    SGPV = .rope_coverage(ci = ci, range_rope, ci_range = final_ci, dof = dof),
     ROPE_low = range_rope[1],
     ROPE_high = range_rope[2],
     ROPE_Equivalence = decision,
@@ -726,8 +740,8 @@ equivalence_test.ggeffects <- function(x,
 # same range / limits as the confidence interval, thus indeed representing a
 # normally distributed confidence interval. We then calculate the probability
 # mass of this interval that is inside the ROPE.
-.rope_coverage <- function(ci = 0.95, range_rope, ci_range) {
-  out <- .generate_posterior_from_ci(ci, ci_range)
+.rope_coverage <- function(ci = 0.95, range_rope, ci_range, dof = Inf) {
+  out <- .generate_posterior_from_ci(ci, ci_range, dof = dof)
   # compare: ci_range and range(out)
   # The SGPV refers to the proportion of the confidence interval inside the
   # full ROPE - thus, we set ci = 1 here
@@ -736,23 +750,39 @@ equivalence_test.ggeffects <- function(x,
 }
 
 
-.generate_posterior_from_ci <- function(ci = 0.95, ci_range, precision = 10000) {
+.generate_posterior_from_ci <- function(ci = 0.95, ci_range, dof = Inf, precision = 10000) {
   # this function creates an approximate normal distribution that covers the
   # CI-range, i.e. we "simulate" a posterior distribution from a frequentist CI
 
-  # first we need the z-values of thq quantiles from the CI range
-  z_value <- stats::qnorm((1 + ci) / 2)
-  # then we need the range of the CI (in units), also to calculate the mean of
+  # sanity check - dof argument
+  if (is.null(dof)) {
+    dof <- Inf
+  }
+  # first we need the range of the CI (in units), also to calculate the mean of
   # the normal distribution
   diff_ci <- abs(diff(ci_range))
   mean_dist <- ci_range[2] - (diff_ci / 2)
+  # then we need the critical values of the quantiles from the CI range
+  z_value <- stats::qt((1 + ci) / 2, df = dof)
   # the range of Z-scores (from lower to upper quantile) gives us the range of
   # the provided interval in terms of standard deviations. now we divide the
   # known range of the provided CI (in units) by the z-score-range, which will
   # give us the standard deviation of the distribution.
   sd_dist <- diff_ci / diff(c(-1 * z_value, z_value))
-  # we now know all parameters (mean and sd) to simulate a normal distribution
-  bayestestR::distribution_normal(n = precision, mean = mean_dist, sd = sd_dist)
+  # generate normal-distribution if we don't have t-distribution, or if
+  # we don't have necessary packages installed
+  if (is.infinite(dof) || !insight::check_if_installed("distributional", quietly = TRUE)) {
+    # tell user to install "distributional"
+    if (!is.infinite(dof)) {
+      insight::format_alert("For models with only few degrees of freedom, install the {distributional} package to increase accuracy of `p_direction()`, `p_significance()` and `equivalence_test()`.") # nolint
+    }
+    # we now know all parameters (mean and sd) to simulate a normal distribution
+    bayestestR::distribution_normal(n = precision, mean = mean_dist, sd = sd_dist)
+  } else {
+    insight::check_if_installed("distributional")
+    out <- distributional::dist_student_t(df = dof, mu = mean_dist, sigma = sd_dist)
+    sort(unlist(distributional::generate(out, times = precision), use.names = FALSE))
+  }
 }
 
 

R/p_significance.R

@@ -35,21 +35,22 @@ bayestestR::p_significance
 #' for functions related to checking effect existence and significance.
 #'
 #' @details `p_significance()` returns the proportion of the _full_ confidence
-#' interval range (assuming a normally distributed, equal-tailed interval) that
-#' is outside a certain range (the negligible effect, or ROPE, see argument
-#' `threshold`). If there are values of the distribution both below and above
-#' the ROPE, `p_significance()` returns the higher probability of a value being
-#' outside the ROPE. Typically, this value should be larger than 0.5 to indicate
-#' practical significance. However, if the range of the negligible effect is
-#' rather large compared to the range of the confidence interval,
-#' `p_significance()` will be less than 0.5, which indicates no clear practical
-#' significance.
+#' interval range (assuming a normally or t-distributed, equal-tailed interval,
+#' based on the model) that is outside a certain range (the negligible effect,
+#' or ROPE, see argument `threshold`). If there are values of the distribution
+#' both below and above the ROPE, `p_significance()` returns the higher
+#' probability of a value being outside the ROPE. Typically, this value should
+#' be larger than 0.5 to indicate practical significance. However, if the range
+#' of the negligible effect is rather large compared to the range of the
+#' confidence interval, `p_significance()` will be less than 0.5, which
+#' indicates no clear practical significance.
 #'
 #' Note that the assumed interval, which is used to calculate the practical
 #' significance, is an estimation of the _full interval_ based on the chosen
 #' confidence level. For example, if the 95% confidence interval of a
-#' coefficient ranges from -1 to 1, the underlying _full (normally distributed)
-#' interval_ approximately ranges from -1.9 to 1.9, see also following code:
+#' coefficient ranges from -1 to 1, the underlying _full (normally or
+#' t-distributed) interval_ approximately ranges from -1.9 to 1.9, see also
+#' following code:
 #'
 #' ```
 #' # simulate full normal distribution
@@ -178,11 +179,12 @@ p_significance.lm <- function(x, threshold = "default", ci = 0.95, verbose = TRU
 .posterior_ci <- function(x, ci, ...) {
   # first, we need CIs
   out <- ci(x, ci = ci, ...)
+  dof <- .safe(insight::get_df(x, type = "wald"), Inf)
   # we now iterate all confidence intervals and create an approximate normal
   # distribution that covers the CI-range.
   posterior <- as.data.frame(lapply(seq_len(nrow(out)), function(i) {
     ci_range <- as.numeric(out[i, c("CI_low", "CI_high")])
-    .generate_posterior_from_ci(ci, ci_range)
+    .generate_posterior_from_ci(ci, ci_range, dof = dof)
   }))
   colnames(posterior) <- out$Parameter