Slashdot has an outstanding report on filtering, and how it works. This is a great read whatever your views on filtering are, read it HERE.
Be sure to check out the link to This Report on sites blocked at the University of UT.
Most measures of blocking software effectiveness focus on how much pornography it blocks. We weren\'t able to test that because we couldn\'t look through the 99.4% of unblocked material - over 53 million URLs. Just too much data. But we did learn that, in Utah, 5% of the time, when the software said \"you can\'t look at that,\" it was just plain wrong.
Ninety-five percent accuracy might sound like a nice high figure to base a good meme around. Who could argue with a number like 95%? But consider what this means for the 300 Web sites in question: each of them was blocked from being read by a great many public institutions in the state of Utah.