diff --git a/src/backend.rs b/src/backend.rs
index 1d28a0873..2339a0b3e 100644
--- a/src/backend.rs
+++ b/src/backend.rs
@@ -59,7 +59,7 @@ pub struct Hop {
     pub worst: Option<Duration>,
     /// The mean duration of the all probes.
     pub mean: f64,
-    ///
+    /// The aggregated squared distance from the mean of all probes.
     pub m2: f64,
     /// The last N `Hop::last` samples.
     pub samples: Vec<Duration>,