Often overlooked, anonymizing data is no trivial endeavor as this post by Bruce Schneier shows.

Last year, Netflix published 10 million movie rankings by 500,000 customers, as part of a challenge for people to come up with better recommendation systems than the one the company was using. The data was anonymized by removing personal details and replacing names with random numbers, to protect the privacy of the recommenders. Arvind Narayanan and Vitaly Shmatikov, researchers at the University of Texas at Austin, de-anonymized some of the Netflix data by comparing rankings and timestamps with public information in the Internet Movie Database , or IMDb.