
Type: Improvement

Status: Closed

Priority: Major

Resolution: Fixed

Affects Version/s: None

Fix Version/s: 0.5.0

Labels:None
There's a small issue which makes it a bit hard to use the NDCG@x
from BinaryResponseMeasure.java
public static double nDCG(@Nonnull final List<?> rankedList, @Nonnull final List<?> groundTruth, @Nonnull final int recommendSize) { double dcg = 0.d; double idcg = IDCG(Math.min(recommendSize, groundTruth.size())); ... public static double IDCG(final int n) { double idcg = 0.d; for (int i = 0; i < n; i++) { idcg += Math.log(2) / Math.log(i + 2); } return idcg; }
You'll notice that the way it calculates the idcg for binary NDCG calculation is that it uses the count in groundTruth. The problem is that when we use "recommendSize" (e.g. NDCG@10) we may pass all the ground Truth and not just the ones in the first 10. This is a bit unexpected. Of course, we could just limit the truths using array intersection and what not, but the users shouldn't really have to do that. You can simply just count the # of matched ground truths so it's easier to use this function.
e.g.
public static double nDCG(@Nonnull final List<?> rankedList, @Nonnull final List<?> groundTruth, @Nonnull final int recommendSize) { double dcg = 0.d; int matchedGroundTruths = 0; for (int i = 0, n = recommendSize; i < n; i++) { Object item_id = rankedList.get(i); if (!groundTruth.contains(item_id)) { continue; } int rank = i + 1; dcg += Math.log(2) / Math.log(rank + 1); matchedGroundTruths++; } double idcg = IDCG(matchedGroundTruths); return dcg / idcg; }
Thanks
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