diff --git a/ch09.jl b/ch09.jl
index 7fc7682..b64068e 100644
--- a/ch09.jl
+++ b/ch09.jl
@@ -157,3 +157,5 @@ ratings_predict = float.(sort(ratings))
 popularity_predict = predict(model, ratings_predict)
 
 plot!(ratings_predict, popularity_predict; width=5, color="black")
+
+combine(groupby(good, :Rating), :Popularity => mean)