statistics: handle stddev of one element

Author: lorenzhs

sampling/benchmark.hpp

@@ -50,9 +50,12 @@ struct statistics {
     double avg() {
         return mean;
     }
-    double stddev() {
-        // assert(count > 1);
-        return sqrt(nvar / (count - 1));
+    double stddev(size_t ddof = 1) {
+        if (count <= 1) return 0.0;
+        // ddof = delta degrees of freedom
+        // Set to 0 if you have the entire distribution
+        // Set to 1 if you have a sample (to correct for bias)
+        return sqrt(nvar / (count - ddof));
     }
 };