diff --git a/ostat.go b/ostat.go
index 002ed27..eeed793 100644
--- a/ostat.go
+++ b/ostat.go
@@ -95,6 +95,8 @@ func (os *OnlineStat) StdDev() float64 {
 	return math.Sqrt(os.Variance())
 }
 
+// CI returns a 95% confidence interval
+// https://en.wikipedia.org/wiki/Confidence_interval
 func (os *OnlineStat) CI() (float64, float64) {
 	// 95% from http://mathworld.wolfram.com/ConfidenceInterval.html
 	conf := 1.95996