7. The recommendation system of claim 1 , further comprising: a data collection engine configured to electronically monitor, substantially in real time, user interactions with a plurality of unique items and to generate data based on the user interactions for use by the penalty computation engine in calculating dissimilarity penalties.
8. The recommendation system of claim 7 , wherein the data collection engine being configured to electronically monitor, substantially in real time, user interactions, comprises being configured to electronically monitor a plurality of item listing services.