Proposition 13 sets the assessed value of properties at the time of purchase (known as an acquisition value system), with a possible 2% annual assessment increase. As a result, properties of equal value can have a great amount of variation in their assessed value, even if they are next to each other. The disparity grows when property prices appreciate by more than 2% a year. The Case–Shiller housing index shows prices in Los Angeles, San Diego, and San Francisco appreciated 170% from 1987 (the start of available data) to 2012 while the 2% cap only allowed a 67% increase in taxes on homes that were not sold during this 26-year period.