http://en.wikipedia.org/wiki/Slope_One Original Python implementation: http://www.serpentine.com/blog/2006/12/12/collaborative-filtering-made-easy/ http://www.serpentine.com/wordpress/wp-content/uploads/2006/12/slope_one.py.txt Differences between Ruby and Python: http://jjinux.blogspot.com/2007/06/ruby-python-programmers-perspective.html