+ double val = data;
+ double delta, n;
+
+ if (data > is->max_val)
+ is->max_val = data;
+ if (data < is->min_val)
+ is->min_val = data;
+
+ delta = val - is->mean;
+ n = is->samples + 1.0;
+ is->mean += delta / n;
+ is->S += delta * (val - is->mean);