diff --git a/av-variance.c b/av-variance.c
index f94ae8b..7a0b8f8 100644
--- a/av-variance.c
+++ b/av-variance.c
@@ -31,6 +31,10 @@ double sample_variance(double set[], int set_n) {
   return ss;
 }
 
+double sample_stddev(double set[], int set_n) {
+  return sqrt(sample_variance(set, set_n));
+}
+
 int main() {
   // Some data
   double d[] = {7, 8, 5, 1, 6, 2};
@@ -49,7 +53,9 @@ int main() {
   assert(isalmost(sample_variance(d, d_n),  7.766, 0.001));
   assert(isalmost(sample_variance(e, e_n), 41.564, 0.001));
 
-  // XXX Stddev
+  assert(isalmost(sample_stddev(d, d_n), 2.787, 0.001));
+  assert(isalmost(sample_stddev(e, e_n), 6.447, 0.001));
+
   // XXX Range
   // XXX Mode
   // XXX Percentile