Add variance to the stats.

src/stats.hpp

@@ -24,7 +24,11 @@ struct Stats {
   }
 
   inline double stddev() {
-    return std::sqrt((sumsq - (sum * sum / n)) / (n - 1));
+    return std::sqrt(variance());
+  }
+
+  inline double variance() {
+    return (sumsq - (sum * sum / n)) / (n - 1);
   }
 
   inline void sample(double s) {