If you want to know the offensive value of Gilbert Arenas or Tyson Chandler, take a look in the box score published in any newspaper. If you want to know their defensive contributions, enter the Twilight Zone of opinion and guesswork. That is, until now.

For years, defensive statistics haven't painted an accurate picture of individual performance. Blocks, steals and defensive rebounds are useful, but don't cover much of what good defenders do. Roland Beech at 82games.com provides useful "on/off" data: measures of what the team does when an individual player is in the lineup vs. when he's not. That information tells a lot about a player's impact, but it doesn't tell us why.

For example, a fan might look up Wizards center Brendan Haywood's on/off information and find the Wizards are nearly 10 points better per 100 possessions defensively when Haywood is on the court. Only five other players with significant playing time -- Tim Duncan, Andrei Kirilenko, Dikembe Mutombo, Jason Collins and Jeff Foster -- have on/off impacts as large. But there's no way to know whether Haywood's "impact" is real or whether it's luck.

To answer that "real vs. luck" question, defensive statistics were collected possession-by-possession and shed considerable light on what individual players are doing on the defensive end. Categories include individual shooting statistics (field-goals allowed and forced misses), forced turnovers that aren't steals and fouls resulting in free throws.