diff --git a/changelog.md b/changelog.md
index 812bed5..e5a69de 100644
--- a/changelog.md
+++ b/changelog.md
@@ -12,6 +12,7 @@
 * Decomission `debug` build flag.
 * Decomission warning when `--timeout` is absent.
 * Add `instance Eq RelStDev` and `instance Ord RelStDev`.
+* Add `unRelStDev :: RelStDev -> Double`.
 
 # 0.3.5
 
diff --git a/src/Test/Tasty/Bench.hs b/src/Test/Tasty/Bench.hs
index 0c7e63a..b2d9868 100644
--- a/src/Test/Tasty/Bench.hs
+++ b/src/Test/Tasty/Bench.hs
@@ -788,7 +788,7 @@ type Progress = ()
 -- only once, use command-line option @--stdev@ @Infinity@.
 --
 -- @since 0.2
-newtype RelStDev = RelStDev Double
+newtype RelStDev = RelStDev { unRelStDev :: Double }
   deriving (Eq, Ord, Show, Read, Typeable)
 
 -- | Whether to measure CPU time or wall-clock time.