diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e1f3597
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,4 @@
+test: test_clock
+	./test_clock
+test_clock:
+	gcc -O3 -o test_clock test_clock.c -lm
diff --git a/test_clock.c b/test_clock.c
index ca7a952..df6a8ba 100644
--- a/test_clock.c
+++ b/test_clock.c
@@ -1,17 +1,43 @@
 #include <stdio.h>
 #include <stdint.h>
 #include <time.h>
+#include <math.h>
 #include "x86intrin.h"
 
+struct rolling_stddev
+{
+    uint64_t sum, count;
+    float mean, m2;
+};
+
+void roll(struct rolling_stddev *r, uint64_t value)
+{
+    r->count++;
+    r->sum += value;
+    float d = value-r->mean;
+    r->mean += d/r->count;
+    r->m2 += d*(value-r->mean);
+}
+
+float dev(struct rolling_stddev *r)
+{
+    return (r->m2/(r->count-1));
+}
+
 void test_clock(const char *name, clockid_t clockid)
 {
-    struct timespec tp, te;
-    clock_gettime(clockid, &te);
+    struct rolling_stddev d = { 0 };
+    struct timespec ts, tp, te;
+    clock_gettime(clockid, &ts);
+    te = ts;
     for (int i = 0; i < 100000000; i++)
     {
         clock_gettime(clockid, &tp);
+        uint64_t cur = ((tp.tv_sec-te.tv_sec)*1000000000 + (tp.tv_nsec-te.tv_nsec));
+        roll(&d, cur);
+        te = tp;
     }
-    printf("%s: %lu ns\n", name, (tp.tv_sec-te.tv_sec)*10 + (tp.tv_nsec-te.tv_nsec)/100000000);
+    printf("%s: %lu ns, stddev %.2f ns\n", name, (te.tv_sec-ts.tv_sec)*10 + (te.tv_nsec-ts.tv_nsec)/100000000, sqrt(dev(&d)));
 }
 
 void test_rdtsc()