One of the stats was was very popular among Statisticians for many
years was/is a system divised originally by Bill James.....
RUNS CREATED.
It's an excellent stat, unique in that it can be used for both pitchers
and batters. 
What the Runs Created stat is actually reflecting is the number of 
runs that would be scored in a game if you were to have an entire 
lineup of that player. Likewise, for a pitcher it is a measure of
how many runs you would give up were that pitcher used against
an entire lineup of "Average" players. 
Because it is based primarily on Hits, Walks, and Total Bases, the
OBPS, or On Base Plus Slugging stat measure essentially the same thing
and is much easier to use. Therefore the use of Runs Created has
diminished. But it is still a very accurate statistic, especially when
used in conjunction with the Strat-O-Matic game. 
The SOM cards are based on a variety of statistics, and the RC numbers
figure them all into the equation.
The use of RCG is extrememly reliable when it come to the Pitcher's
cards. ERA doesn't account for the number of hits, walks, homers, ect.
It only accounts for the runers that score without the help of errors.
While a Major League pitcher may be adept at spreading the hits and
walks around from inning to inning, enabling them to keep their ERA
low, the SOM game cannot determine WHEN the hits will occur, just 
the NUMBER of them that will occur. 
In 9 innings, if a pitcher gives up a walk, a single, and a 3 run 
homer in inning 3, and another 3 hits during the game, one being
a 2-run homer, the pitcher will have an ERA of 5.00.... not that good.
Another pitcher gives up a 5 singles, 1 double and 2 walks in the first
8 innings without anyone scoring, then gives up a double and a solo homer
in the 9th.
He has an ERA of 2.00 which is excellent.
The SOM game will reflect the number of hits and homers, but naturally
cannot determine when they will occur. So the unlucky pitcher in the
first example will not likely group his hits and homers together, nor
will the lucky guy in the second example be able to spread all the 
hits out like that.

How does this work?

 

Runs Created =  (H+BB)*TB/(AB+BB)
Meaning "Hits + Walks, X Total Bases Divided by, ABs + Walks"

Runs Created per Game = (RC/(AB-H)*25)
Meaning "Runs Created divided by, ABs - Hits, X 25

Using the two simple examples for the pitchers above, the applying
the RC formula gives you these results:

Pitcher 1 Has 1 walk, 3 singles, and 1 home run. This is 7 Total Bases.
The pitcher, having gone 9 innings has recorded 27 outs and 4 hits 
which means he faced 31 ABs and 1 walk. 
So  (H+BB) X TB  /(AB+BB)   
     (4+1) X  7  /(31+1)  = 35 divided by 32 for a RC of 1.09

Pitcher 2 Has 2 walks, 6 singles, 1 double, and 1 home run. This is
12 Total Bases. The ABs is 35 with 2 walks.
So  (H+BB) X TB  /(AB+BB)
     (8+2) X 12  /(35+2)  = 120 divided by 37 for a RC of 3.24

CONCLUSION:  The ERAs in the case of these pitchers would be very
deceiving. The SOM cards would reflect the number of hits, including
doubles and homers given up. The first pitcher would end up with a 
much better card and the second pitcher, while not all that bad, 
would not have a card as good as his ERA would suggest.

For batters, it's a much simpler matter, and the primary benefit of
using the RC is to compare players that have varying numbers and 
varying strengths.

In actual use, the Runs Created per Game number is actually preferred.
The reason is that since everyone is playing a different number of
games, their total runs created will be dramatically different from
each other and of little use. 
The Runs Created number for batters can be used in the same sense
as RBI's, so 100 for a full-time player is a good number.
For RCG, it's a little less cut and dried as to what deteremines what
is good. Knowing the league average is a nice starting point, but even
then the player's position makes a big difference. A RCG of 5.0 is
very good for a SS, C, or 2B, but mediocre for a CF, and downright
poor for a 1B. 
In baseball leagues with 18 or 20 teams, players should be used
that are at the Major League average or better. For pitchers, most of
the staff should be better than the Major League average, though 
using one or two worse than the average can be expected.

In setting up the RC and RCG on a spreadsheet, you need to assemble 
the numbers for HITS and WALKS which are readily available. You also
need to include 2B, 3B and HR in order to get the TB number for
TOTAL BASES. I then include a column for TB with the formula
(H)+(2B)+(3B*2)+(HR*3). 
Once the RC number is determined, the RCG can be figured out.

Pitchers are generally a little bit tougher to figure out since 
the stats needed aren't as readily available. You need either their
total bases or the 2B, 3B and HR numbers that they've given up.
You also need the Opposing Batting Avg in order to determine 
the number of ABs they've faced. Normally to determine Batting Avg,
you divide H by ABs. To find the ABs then, you divide hits by the
batting avg.

Finally, I always figure out the RCG for batters vs LHP and RHP
seperately since this is a large part of the SOM game. But for
pitchers I don't bother. Starting pitchers will face batters that
hit either way so it doesn't matter where their stengths lie. For
relievers it may make a difference, but the game's ratings tell you
when they are best used.