diff --git a/osu.Game/Rulesets/Scoring/HitEventExtensions.cs b/osu.Game/Rulesets/Scoring/HitEventExtensions.cs
index 5c0b4ecac3..637d0a872a 100644
--- a/osu.Game/Rulesets/Scoring/HitEventExtensions.cs
+++ b/osu.Game/Rulesets/Scoring/HitEventExtensions.cs
@@ -22,6 +22,13 @@ namespace osu.Game.Rulesets.Scoring
             return 10 * standardDeviation(timeOffsets);
         }
 
+        /// <summary>
+        /// Calculates the average hit offset/error for a sequence of <see cref="HitEvent"/>s, where negative numbers mean the user hit too early on average.
+        /// </summary>
+        /// <returns>
+        /// A non-null <see langword="double"/> value if unstable rate could be calculated,
+        /// and <see langword="null"/> if unstable rate cannot be calculated due to <paramref name="hitEvents"/> being empty.
+        /// </returns>
         public static double? CalculateAverageHitError(this IEnumerable<HitEvent> hitEvents) =>
             hitEvents.Where(affectsUnstableRate).Select(ev => ev.TimeOffset).Average();