diff --git a/code/statistics.py b/code/statistics.py
index 9db936c6..c80d3c05 100644
--- a/code/statistics.py
+++ b/code/statistics.py
@@ -47,7 +47,7 @@ def median(v):
         
 def quantile(x, p):
     """returns the pth-percentile value in x"""
-    p_index = int(p * len(x))
+    p_index = int(p * len(x)) - 1 # for sorted list x is indexed from 0 to len(x)-1
     return sorted(x)[p_index]
 
 def mode(x):
@@ -132,4 +132,4 @@ def correlation(x, y):
     print "covariance(num_friends, daily_minutes)", covariance(num_friends, daily_minutes)
     print "correlation(num_friends, daily_minutes)", correlation(num_friends, daily_minutes)
     print "correlation(num_friends_good, daily_minutes_good)", correlation(num_friends_good, daily_minutes_good)
-    
\ No newline at end of file
+