[MAHOUT-1144] Wrong normalization in SVD++

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.7
• Fix Version/s: 0.8
• Labels: svd svd++
• Assignee: Sebastian Schelter
• Reporter: Sebastian Schelter
• Created: 31/Jan/13 11:15
• Updated: 31/Jan/24 22:11
• Resolved: 11/Mar/13 16:18

Description

Reported by Agnonchik:

Seems that I have found a discrepancy of this implementation from the original Yehuda Koren's SVD++ algorithm.

line 140:
double denominator = Math.sqrt(itemsByUser.size());
should be
double denominator = Math.sqrt(itemsByUser.get(u).size());
line 164:
double denominator = Math.sqrt(itemsByUser.size());
should be
double denominator = Math.sqrt(itemsByUser.get(u).size());

The sum of y parameters should be normalized by square root of number of items for which user u provided implicit feedback. Am I right?
Currently, it is normalized by square root of number of users not items.