add fairness metrics

NAMESPACE

@@ -130,10 +130,13 @@ export(concordance_survival_vec)
 export(conf_mat)
 export(curve_metric_summarizer)
 export(curve_survival_metric_summarizer)
+export(demographic_parity)
 export(detection_prevalence)
 export(detection_prevalence_vec)
 export(dots_to_estimate)
 export(dynamic_survival_metric_summarizer)
+export(equal_opportunity)
+export(equalized_odds)
 export(f_meas)
 export(f_meas_vec)
 export(finalize_estimator)
@@ -174,6 +177,7 @@ export(msd)
 export(msd_vec)
 export(new_class_metric)
 export(new_dynamic_survival_metric)
+export(new_groupwise_metric)
 export(new_integrated_survival_metric)
 export(new_numeric_metric)
 export(new_prob_metric)

NEWS.md

@@ -14,6 +14,8 @@ calculated with `roc_auc_survival()`.
 
 * `metric_set()` can now be used with a combination of dynamic and static survival metrics.
 
+* `demographic_parity()`, `equalized_odds()`, and `equal_opportunity()` are new metrics for measuring model fairness. Each is implemented with the `new_groupwise_metric()` constructor, a general interface for defining group-aware metrics that allows for quickly and flexibly defining fairness metrics with the problem context in mind.
+
 # yardstick 1.2.0
 
 ## New Metrics

R/metrics.R → R/aaa-metrics.R

@@ -624,6 +624,19 @@ validate_function_class <- function(fns) {
     }
   }
 
+  # Special case unevaluated group-wise metric factories
+  if ("metric_factory" %in% fn_cls) {
+    factories <- fn_cls[fn_cls == "metric_factory"]
+    cli::cli_abort(
+      c("{cli::qty(factories)}The input{?s} {.arg {names(factories)}} \\
+         {?is a/are} {.help [group-wise metric](yardstick::new_groupwise_metric)} \\
+         {?factory/factories} and must be passed a data-column before
+         addition to a metric set.",
+        "i" = "Did you mean to type e.g. `{names(factories)[1]}(col_name)`?"),
+      call = rlang::call2("metric_set")
+    )
+  }
+
   # Each element of the list contains the names of the fns
   # that inherit that specific class
   fn_bad_names <- lapply(fn_cls_unique, function(x) {

R/aaa.R

@@ -6,6 +6,7 @@ utils::globalVariables(
   c(
     # for class prob metrics
     "estimate",
+    ".estimator",
     "threshold",
     "specificity",
     ".level",

R/fair-aaa.R

@@ -0,0 +1,221 @@
+#' Create group-wise metrics
+#'
+#' Group-wise metrics quantify the disparity in value of a metric across a
+#' number of groups. Group-wise metrics with a value of zero indicate that the
+#' underlying metric is equal across groups. yardstick defines
+#' several common fairness metrics using this function, such as
+#' [demographic_parity()], [equal_opportunity()], and [equalized_odds()].
+#'
+#' Note that _all_ yardstick metrics are group-aware in that, when passed
+#' grouped data, they will return metric values calculated for each group.
+#' When passed grouped data, group-wise metrics also return metric values
+#' for each group, but those metric values are calculated by first additionally
+#' grouping by the variable passed to `by` and then summarizing the per-group
+#' metric estimates across groups using the function passed as the
+#' `aggregrate` argument.
+#'
+#' @param fn A yardstick metric function or metric set.
+#' @param name The name of the metric to place in the `.metric` column
+#' of the output.
+#' @param aggregrate A function to summarize the generated metric set results
+#' `x`. In many cases, `~diff(range(x$.estimate))` or
+#' `~r <- range(x$.estimate); r[1]/r[2]`.
+#' @inheritParams new_class_metric
+#'
+#' @section Relevant Group Level:
+#' Additional arguments can be passed to the function outputted by
+#' the function that this function outputs. That is:
+#'
+#' ```
+#' res_fairness <- new_groupwise_metric(...)
+#' res_by <- res_fairness(by)
+#' res_by(..., additional_arguments_to_aggregrate = TRUE)
+#' ```
+#'
+#' For finer control of how groups in `by` are treated, use the
+#' `aggregrate` argument.
+#'
+#' @return
+#' This function is a
+#' [function factory](https://adv-r.hadley.nz/function-factories.html); it's
+#' output is itself a function. Further, the functions that this function
+#' outputs are also function factories. More explicitly, this looks like:
+#'
+#' ```
+#' # a function with similar implementation to `demographic_parity()`:
+#' diff_range <- function(x) {diff(range(x$.estimate))}
+#'
+#' dem_parity <-
+#'   new_groupwise_metric(
+#'     fn = detection_prevalence,
+#'     name = "dem_parity",
+#'     aggregrate = diff_range
+#'   )
+#' ```
+#'
+#' The outputted `dem_parity` is a function that takes one argument, `by`,
+#' indicating the data-masked variable giving the sensitive feature.
+#'
+#' When called with a `by` argument, `dem_parity` will return a yardstick
+#' metric function like any other:
+#'
+#' ```
+#' dem_parity_by_gender <- dem_parity(gender)
+#' ```
+#'
+#' Note that `dem_parity` doesn't take any arguments other than `by`, and thus
+#' knows nothing about the data it will be applied to other than that it ought
+#' to have a column with name `"gender"` in it.
+#'
+#' The output `dem_parity_by_gender` is a metric function that takes the
+#' same arguments as the function supplied as `fn`, in this case
+#' `detection_prevalence`. It will thus interface like any other yardstick
+#' function except that it will look for a `"gender"` column in
+#' the data it's supplied.
+#'
+#' In addition to the examples below, see the documentation on the
+#' return value of fairness metrics like [demographic_parity()],
+#' [equal_opportunity()], or [equalized_odds()] to learn more about how the
+#' output of this function can be used.
+#'
+#' @examples
+#' data(hpc_cv)
+#'
+#' # `demographic_parity`, among other fairness metrics,
+#' # is generated with `new_groupwise_metric()`:
+#' diff_range <- function(x) {diff(range(x$.estimate))}
+#' demographic_parity_ <-
+#'   new_groupwise_metric(
+#'     fn = detection_prevalence,
+#'     name = "demographic_parity",
+#'     aggregrate = diff_range
+#'   )
+#'
+#' m_set <- metric_set(demographic_parity_(Resample))
+#'
+#' m_set(hpc_cv, truth = obs, estimate = pred)
+#'
+#' # the `post` argument can be used to accommodate a wide
+#' # variety of parameterizations. to encode demographic
+#' # parity as a ratio inside of a difference, for example:
+#' ratio_range <- function(x, ...) {
+#'   range <- range(x$.estimate)
+#'   range[1] / range[2]
+#' }
+#'
+#' demographic_parity_ratio <-
+#'   new_groupwise_metric(
+#'     fn = detection_prevalence,
+#'     name = "demographic_parity_ratio",
+#'     aggregrate = ratio_range
+#'   )
+#'
+#' @export
+new_groupwise_metric <- function(fn, name, aggregrate, direction = "minimize") {
+  if (is_missing(fn) || !inherits_any(fn, c("metric", "metric_set"))) {
+    abort("`fn` must be a metric function or metric set.")
+  }
+  if (is_missing(name) || !is_string(name)) {
+    abort("`name` must be a string.")
+  }
+  if (is_missing(aggregrate) || !is_function(aggregrate)) {
+    abort("`aggregrate` must be a function.")
+  }
+  arg_match(
+    direction,
+    values = c("maximize", "minimize", "zero")
+  )
+
+  metric_factory <-
+    function(by) {
+      by_str <- as_string(enexpr(by))
+      res <-
+        function(data, ...) {
+          gp_vars <- dplyr::group_vars(data)
+
+          if (by_str %in% gp_vars) {
+            cli::cli_abort(
+              "Metric is internally grouped by {.field {by_str}}; grouping \\
+              {.arg data} by {.field {by_str}} is not well-defined."
+            )
+          }
+
+          # error informatively when `fn` is a metric set; see `eval_safely()`
+          data_grouped <- dplyr::group_by(data, {{by}}, .add = TRUE)
+          res <-
+            tryCatch(
+              fn(data_grouped, ...),
+              error = function(cnd) {
+                if (!is.null(cnd$parent)) {
+                  cnd <- cnd$parent
+                }
+
+                abort(conditionMessage(cnd), call = call(name))
+              }
+            )
+
+          # restore to the grouping structure in the supplied data
+          if (length(gp_vars) > 0) {
+            res <- dplyr::group_by(res, !!!dplyr::groups(data), .add = FALSE)
+          }
+
+          group_rows <- dplyr::group_rows(res)
+          group_keys <- dplyr::group_keys(res)
+          res <- dplyr::ungroup(res)
+          groups <- vec_chop(res, indices = group_rows)
+          out <- vector("list", length = length(groups))
+
+          for (i in seq_along(groups)) {
+            group <- groups[[i]]
+
+            .estimate <- aggregrate(group)
+
+            if (!is_bare_numeric(.estimate)) {
+              abort(
+                "`aggregrate` must return a single numeric value.",
+                call = call2("new_groupwise_metric")
+              )
+            }
+
+            elt_out <- list(
+              .metric = name,
+              .by = by_str,
+              .estimator = group$.estimator[1],
+              .estimate = .estimate
+            )
+
+            out[[i]] <- tibble::new_tibble(elt_out)
+          }
+
+          group_keys <- vctrs::vec_rep_each(group_keys, times = list_sizes(out))
+          out <- vec_rbind(!!!out)
+          out <- vec_cbind(group_keys, out)
+
+          out
+        }
+      res <- new_class_metric(res, direction = "minimize")
+
+      structure(
+        res,
+        direction = direction,
+        by = by_str,
+        class = groupwise_metric_class(fn)
+      )
+    }
+
+  structure(metric_factory, class = c("metric_factory", "function"))
+}
+
+groupwise_metric_class <- function(fn) {
+  if (inherits(fn, "metric")) {
+    return(class(fn))
+  }
+
+  class(attr(fn, "metrics")[[1]])
+}
+
+diff_range <- function(x) {
+  estimates <- x$.estimate
+
+  max(estimates) - min(estimates)
+}

R/fair-demographic_parity.R

@@ -0,0 +1,50 @@
+#' Demographic parity
+#'
+#' @description
+#' Demographic parity is satisfied when a model's predictions have the
+#' same predicted positive rate across groups. A value of 0 indicates parity
+#' across groups. Note that this definition does not depend on the true
+#' outcome; the `truth` argument is included in outputted metrics
+#' for consistency.
+#'
+#' `demographic_parity()` is calculated as the difference between the largest
+#' and smallest value of [detection_prevalence()] across groups.
+#'
+#' Demographic parity is sometimes referred to as group fairness,
+#' disparate impact, or statistical parity.
+#'
+#' See the "Measuring Disparity" section for details on implementation.
+#'
+#' @param by The column identifier for the sensitive feature. This should be an
+#' unquoted column name referring to a column in the un-preprocessed data.
+#'
+#' @templateVar fn demographic_parity
+#' @templateVar internal_fn detection_prevalence
+#' @templateVar internal_fn [detection_prevalence()]
+#' @template return-fair
+#' @template event-fair
+#' @template examples-fair
+#'
+#' @family fairness metrics
+#'
+#' @references
+#'
+#' Agarwal, A., Beygelzimer, A., Dudik, M., Langford, J., & Wallach, H. (2018).
+#' "A Reductions Approach to Fair Classification." Proceedings of the 35th
+#' International Conference on Machine Learning, in Proceedings of Machine
+#' Learning Research. 80:60-69.
+#'
+#' Verma, S., & Rubin, J. (2018). "Fairness definitions explained". In
+#' Proceedings of the international workshop on software fairness (pp. 1-7).
+#'
+#' Bird, S., Dudík, M., Edgar, R., Horn, B., Lutz, R., Milan, V., ... & Walker,
+#' K. (2020). "Fairlearn: A toolkit for assessing and improving fairness in AI".
+#' Microsoft, Tech. Rep. MSR-TR-2020-32.
+#'
+#' @export
+demographic_parity <-
+  new_groupwise_metric(
+    fn = detection_prevalence,
+    name = "demographic_parity",
+    aggregrate = diff_range
+  )

R/fair-equal_opportunity.R

@@ -0,0 +1,46 @@
+#' Equal opportunity
+#'
+#' @description
+#'
+#' Equal opportunity is satisfied when a model's predictions have the same
+#' true positive and false negative rates across protected groups. A value of
+#' 0 indicates parity across groups.
+#'
+#' `equal_opportunity()` is calculated as the difference between the largest
+#' and smallest value of [sens()] across groups.
+#'
+#' Equal opportunity is sometimes referred to as conditional procedure accuracy
+#' equality or disparate mistreatment.
+#'
+#' See the "Measuring Disparity" section for details on implementation.
+#'
+#' @inheritParams demographic_parity
+#'
+#' @templateVar fn equal_opportunity
+#' @templateVar internal_fn sens
+#' @templateVar internal_fn [sens()]
+#' @template return-fair
+#' @template event-fair
+#' @template examples-fair
+#'
+#' @family fairness metrics
+#'
+#' @references
+#'
+#' Hardt, M., Price, E., & Srebro, N. (2016). "Equality of opportunity in
+#' supervised learning". Advances in neural information processing systems, 29.
+#'
+#' Verma, S., & Rubin, J. (2018). "Fairness definitions explained". In
+#' Proceedings of the international workshop on software fairness (pp. 1-7).
+#'
+#' Bird, S., Dudík, M., Edgar, R., Horn, B., Lutz, R., Milan, V., ... & Walker,
+#' K. (2020). "Fairlearn: A toolkit for assessing and improving fairness in AI".
+#' Microsoft, Tech. Rep. MSR-TR-2020-32.
+#'
+#' @export
+equal_opportunity <-
+  new_groupwise_metric(
+    fn = sens,
+    name = "equal_opportunity",
+    aggregrate = diff_range
+  )