Since the spring training games are about to start, I thought I'd discuss the relevance of spring training statistics.  My contention is that they are meaningless.  I suppose it would be more accurate to say we don't know how meaningful any spring training stat is.  The real point is that they are completely unreliable and don't really tell us anything about the player or how well he'll do in the regular season for the following reasons:

1. Sample size.  Every player's official spring training stats represent a very small sample size.  For starting pitchers, this means 15-25 IP.  For relievers, 5-15 IP (or less).  For position players, this means 40-70 AB (regulars) and even less for backups.  Actual spring training "A games" are only a small part of the work players do in spring training.  There are also "B games," minor league games, simulated games and of course all of the practices and workouts.
2. Uneven competition.  There is a wide variety of talent in spring training.  A player will end up facing some complete scrubs, some ok minor leaguers, some decent major leaguers and some great major leaguers.  When looking at a player's spring training stats, you never know if he faced a disproportionate number of good or bad players.  The level of competition changes throughout spring training.  There are more bad players early in spring training and they are weeded out as spring training progresses.  Also, early in spring training, the pitchers are usually said to be ahead of the hitters.  So spring training stats also can reflect when a player got his playing time and thus, the quality of players he played against.
3. Unreal play.  Not every player in spring training is always playing as if it were a real game.  And I'm not just talking about the veterans who cruise through spring training and don't give 100% because they know they have a starting spot locked up.  There are also many games throughout spring training when pitchers are working on particular pitches, so they throw almost all fastballs, or don't throw one of their breaking pitches to decrease strain on their arms.  Position players also might be working on contact in some games, or bunting or some other particular skill.  All of these things are done to work on one element of a player's game without regard to actual game outcome or the player's stats.
4. Rarefied air.  For a variety of reasons, the ball carries very well in Arizona.  That can pump up power numbers and hurt pitching stats.

These are some of the reasons why spring training stats are unreliable and essentially meaningless.  There are many examples to prove this.  Here are several just from the last two seasons of Royals baseball (with their spring training (ST) and regular season (RS) stats):

Brian Bannister 2007
ST - 10.03 ERA
RS - 3.87 ERA

Gil Meche 2007
ST - 7.31 ERA
RS - 3.67 ERA

Ryan Braun 2007
ST - 3.48 ERA
RS - 6.64 ERA

Jason Standridge 2007
ST - 4.35 ERA
RS - 8.22 ERA

Denny Bautista 2006
ST - 3.00 ERA
RS - 5.62 ERA

Mike Wood 2006
ST - 2.50 ERA
RS - 5.71 ERA

Ambiorix Burgos 2006
ST - 3.46 ERA
RS - 5.52 ERA

Steve Andrade
ST - 1.64 ERA
RS - 9.64 ERA

J.P. Howell 2006
ST - 4.00 ERA
RS - 5.10 ERA

Mark Teahen 2007
ST - 329/385/557
RS - 285/353/410

Ross Gload 2007
ST - 393/424/643
RS - 288/318/441

Fernando Cortez 2007
ST - 429/500/464
RS - 286/333/357

Alex Gordon 2007
ST - 317/419/556
RS - 247/314/411

Mark Grudzielanek 2007
ST - 286/333/286
RS - 302/346/426

Angel Berroa 2006
ST - 439/439/684
RS - 234/259/333

Doug Mientkiewicz 2006
ST - 431/500/627
RS - 283/359/411

John Buck 2006
ST - 128/163/234
RS - 245/306/396

Joe McEwing 2006
ST - 436/500/667
RS - 000/000/000 (6 ab)

Matt Stairs 2006
ST - 132/214/132
RS - 261/352/429

You can see many huge differences from spring training to the regular season.  Sometimes they were much better in spring training and sometimes they were much worse.  And this is the whole point with spring training stats.  Sometimes they will be indicative of what the player does in the regular season.  But often they will differ greatly.  You just never know.

And yet, fans on this site and all over the country will say throughout March that Player X should make the team, or be a starter or make the rotation or be traded or released because of his spring stats.  That makes no sense to me.  Spring training stats aren't good or reliable evidence of anything.  So, how should roster decisions be made going into the regular season?  As I said above, A games represent a small percentage of spring training work.  An organization should (and does) make its personnel decisions based on what the players do in all kinds of spring training games, as well as how they have looked all spring long in practices and workouts.  And, of course, performance in prior years is an extremely important element in evaluating these players.  I'm all for evaluating players based on performance more than simply tools or skills, but there just isn't enough reliably measureable performance in spring training for us to just go by the stats.

So, let's have a fun spring training and hope no one gets hurt.  But let's not hang our hats on spring training stats.  (rhyme unintended)