diff --git a/education/statistics/Sampling.md b/education/statistics/Sampling.md
index 0f2bc21..c22e7c9 100644
--- a/education/statistics/Sampling.md
+++ b/education/statistics/Sampling.md
@@ -34,3 +34,5 @@ An ideal sample will represent the whole population.
 | Response | When the question is worded in a leading way to elicit a certain response. |
 | Volunteer response | Self selecting, individuals volunteer to answer |
 | Measurement | Interviewing method influences the response, uses loaded words or ambiguities. |
+(Ch 20, stat 1040)
+