diff --git a/src/metrics/mod.rs b/src/metrics/mod.rs
index 74f85c6..ea97102 100644
--- a/src/metrics/mod.rs
+++ b/src/metrics/mod.rs
@@ -1,4 +1,4 @@
-use crate::stats::EnumerableCategory;
+use crate::stats::{Categorical, EnumerableCategory};
 use std::fmt;
 
 /// [ResponseStatusCode] groups HTTP response status codes according
@@ -18,6 +18,18 @@ pub enum ResponseStatusCode {
     _5XX,
 }
 
+impl Categorical<5> for ResponseStatusCode {
+    fn category(&self) -> usize {
+        match self {
+            Self::_1XX => 0,
+            Self::_2XX => 1,
+            Self::_3XX => 2,
+            Self::_4XX => 3,
+            Self::_5XX => 4,
+        }
+    }
+}
+
 impl fmt::Display for ResponseStatusCode {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
diff --git a/src/stats/categorical.rs b/src/stats/categorical.rs
new file mode 100644
index 0000000..f0f2223
--- /dev/null
+++ b/src/stats/categorical.rs
@@ -0,0 +1,43 @@
+/// Data is [Categorical] if each element has a surjective mapping to a number
+/// from `[0, N)`. An `[Categorical]` trait expresses data that fits into exactly one
+/// of `N` categories (or bins). The value of `N` represents the total (i.e. the max)
+/// number of categories.
+/// For example, if modeling bools, the groups are `True` and `False`, so N=2.
+/// If modeling a six sided die, the groups would be 0 through 5, so N=6.
+/// Each instance must be able to report which category it belongs to (using Self::category method).
+/// Categories are zero-indexed (the first category is represented by `0usize`).
+/// You can think of an [EnumerableCategory] as a hashmap with fixed integer keys. When the map is
+/// created, its keys must already be known and completely cover the range `[0, N)`.
+///
+/// ```rust
+/// use std::collections::HashSet;
+/// use canary::stats::Categorical;
+///
+/// #[derive(PartialEq, Eq, Debug, Hash)]
+/// enum Coin {
+///   Heads,
+///   Tails,
+/// }
+///
+/// impl Categorical<2> for Coin {
+///     fn category(&self) -> usize {
+///         match self {
+///             Self::Heads => 0,
+///             Self::Tails => 1,
+///         }
+///     }
+/// }
+/// ```
+pub trait Categorical<const N: usize> {
+    fn category(&self) -> usize;
+}
+
+#[cfg(test)]
+mod tests {
+    use static_assertions::assert_obj_safe;
+
+    use super::Categorical;
+
+    // The categorical trait must be object-save.
+    assert_obj_safe!(Categorical<5>);
+}
diff --git a/src/stats/chi.rs b/src/stats/chi.rs
index 9bfa540..dee5e7a 100644
--- a/src/stats/chi.rs
+++ b/src/stats/chi.rs
@@ -25,13 +25,15 @@ fn degrees_of_freedom<Cat: EnumerableCategory>(table: &impl ContingencyTable<Cat
 /// created, its keys must already be known and initialized with zero values.
 ///
 /// ```rust
+/// use std::collections::HashSet;
+/// use canary::stats::EnumerableCategory;
+///
 /// #[derive(PartialEq, Eq, Debug, Hash)]
 /// enum Coin {
 ///   Heads,
 ///   Tails,
 /// }
-/// use std::collections::HashSet;
-/// use canary::stats::EnumerableCategory;
+///
 /// impl EnumerableCategory for Coin {
 ///     fn groups() -> Box<dyn Iterator<Item = Self>> {
 ///         Box::new([Coin::Heads, Coin::Tails].into_iter())
diff --git a/src/stats/mod.rs b/src/stats/mod.rs
index 8e7fdde..bd2a6b6 100644
--- a/src/stats/mod.rs
+++ b/src/stats/mod.rs
@@ -1,5 +1,6 @@
 use std::collections::HashMap;
 
+pub use categorical::Categorical;
 pub use chi::EnumerableCategory;
 pub use group::Group;
 pub use observation::{CategoricalObservation, Observation};
@@ -99,6 +100,8 @@ impl ChiSquareEngine {
 /// This type maps the dependent variable to its count.
 type Table = HashMap<ResponseStatusCode, usize>;
 
+/// For modeling categorical data.
+mod categorical;
 /// contains the engine to calculate the chi square test statistic.
 mod chi;
 /// `group` defines the two groups.