--- a/modules/simulation/random.test	2026-02-26 23:07:06.436861304 +0100
+++ b/modules/simulation/random.test	2026-02-26 23:12:52.124392444 +0100
@@ -167,7 +167,7 @@
         set stdev [expr {$stdev + $rnd * $rnd}]
     }
     set mean  [expr {$mean  / 1000.0}]
-    set stdev [expr {sqrt($stdev / 1000.0)}]
+    set stdev [expr {sqrt($stdev / 1000.0 - $mean * $mean)}]
 
     #
     # We use a rough estimate for the deviation in the mean and stdev