diff --git a/nonpers-assignment/src/main/java/org/lenskit/mooc/nonpers/assoc/BasicAssociationModelProvider.java b/nonpers-assignment/src/main/java/org/lenskit/mooc/nonpers/assoc/BasicAssociationModelProvider.java
index 415941b..190eda7 100644
--- a/nonpers-assignment/src/main/java/org/lenskit/mooc/nonpers/assoc/BasicAssociationModelProvider.java
+++ b/nonpers-assignment/src/main/java/org/lenskit/mooc/nonpers/assoc/BasicAssociationModelProvider.java
@@ -62,6 +62,7 @@ public class BasicAssociationModelProvider implements Provider<AssociationModel>
         for (Long2ObjectMap.Entry<LongSortedSet> xEntry: itemUsers.long2ObjectEntrySet()) {
             long xId = xEntry.getLongKey();
             LongSortedSet xUsers = xEntry.getValue();
+            HashSet xSet = new HashSet(xUsers);
 
             // set up a map to hold the scores for each 'y' item for this 'x'
             Long2DoubleMap itemScores = new Long2DoubleOpenHashMap();
@@ -72,9 +73,9 @@ public class BasicAssociationModelProvider implements Provider<AssociationModel>
                 LongSortedSet yUsers = yEntry.getValue();
 
                 // TODO Compute P(Y & X) / P(X) and store in itemScores
-                HashSet ySet = new HashSet(yUsers), xSet = new HashSet(xUsers);
+                HashSet ySet = new HashSet(yUsers);
                 ySet.retainAll(xSet);
-                double p = ySet.size() / xSet.size();
+                double p = 1.0 * ySet.size() / xUsers.size();
                 itemScores.put(yId, p);
             }