I went over the basics of offensive and defensive efficiency last week, and I'd intended to use this piece to go over Dean Oliver's four factors, but The Only Colors has already taken care of that with this handy primer. Instead, I'm going to talk more generally about some key concepts at the team and individual levels.
1.) Interpreting Offensive Rating properly.
Offensive Rating on the individual level is essentially the same concept as offensive efficiency on the team level--it's derived by taking the number of points a player produces and dividing that by the possessions a player uses, multiplied by 100.
While this metric provides an accurate overview of a player's contributions, it should never be used in the "player A is better than player B because player A's ORtg is higher" sense. Offensive Rating is highly contextual. It's a reflection of a player's talent, but it's also heavily influenced by how a player fits into his team--how good his teammates are, how well they complement his skills, how the team's system complements his skills, etc. All sorts of external factors affect a guy's efficiency, and those factors can vary in number and impact team-to-team.
2.) Usage and efficiency.
%Poss, or usage, defines a player's workload. Ken's scouting reports are helpful here as well; you can see how players are categorized based on their usages (go-to guy, role player, etc).
Since there are five guys on the court, an average player uses 20% of the team's possessions. Players that use more than 20% of the possessions are usually the star players/primary scoring options. Guys who use less than 20% of the possessions are the role players, the guys who have more limited skill sets and need help from teammates to create their own offense. Your Dennis Horners, basically.
Generally speaking, the more possessions a player uses, the less efficient he is. In order to properly frame a guy's ORtg, then, it's essential to use that metric in tandem with %Poss. Two players with the same ORtg on the same team may appear equally valuable, but if one is using 24% of the possessions while the other is using 16%, it's clear whose contributions are more vital. This ties back into #1.
3.) Is that good or bad? One of the hurdles associated with any new concept is getting the proper feel for it. Okay, NC State turned the ball over on 21.5% of its possessions last year. Tracy Smith grabbed 17.4% of the Pack's offensive rebounds while he was on the court. Is that good, bad, just okay? The easiest way to grow comfortable with these and other tempo-free stats is to peruse the various leaderboards provided by Ken's site.
4.) Every rebound needs an adjective. It's surprising how many teams rebound well at one end but poorly or merely adequately at the other. Offensive and defensive rebounding are two distinct skills, and the former can be significantly affected by a coach's philosophy. They should be approached separately. Further, rebounding opportunities are subject not just to varying tempos but also to the game-to-game whims of shooting percentage. Friends don't let friends cite rebounding margin. With your help, we can end this terrible plague.