i agree with dean, the +/- "ratio" (as "they" call it) is in my opinion
useless. yet a number of nba teams use it (IBM "data mining" techniques
extract most of it). the bottom line is what good is a part time player with
a very positive +/- on a team where most of the starters have negative +/-
(or vice versa). the bottom line is who plays at the same time with whom, and
as far as i know that info is not closely tracked by nba teams. obviously the
teams with high W-L % will have starters with a high +/- ratio and those
teams with a low W-L % will have a low +/-...
in hockey its one thing because players tend to play with specific "lines",
but not so in the nba...
i do have pollack's guides from 88-89 to 96-97, some of which have the +/-,
and i've looked closely at the listings of +/- without being able to discern
any appreciable pattern...
as for how often players guard their "counterparts" the answer is well over
50% of the time (meaning in seconds and minutes), and my guess is 80% of the
time if not more (i used to watch hundreds of nba games on tape over a decade
ago). however, the problem is that those defensive statistics that the league
keeps track of (steals, blocks, charging, fouls) occur much more often when
an offensive player loses his man and is then guarded by one of the other 4
opponents. players who block 200-300 or more shots a year block all 5
opponents shots, not just the man he is directly guarding, many many fouls
occur on switches, and i'd guess most steals are not by the man directly
guarding the player with the ball but made on pass attempts...