figuring out an earned run average for a pitcher

One of the best measures of the effectiveness of a pitcher is their earned run average, or as it is commonly called,  an ERA.  The ERA is the average amount of earned runs a pitcher will give up if they pitch 9 innings, or a complete baseball game.

So how do you calculate an ERA?  I've always created a simple proportion to solve this.  For instance, Twins pitcher, Francisco Liriano pitched 3 innings Wednesday and allowed 7 earned runs.  You take the 7 earned runs x 9 innings in a regulation game, divided by the number of innings pitched, which was 3.  With that, you get Liriano's ERA of 21.00 for today's game.

Francisco Liriano did not have a good Wednesday

Twenty-one is not a good ERA.  Today's performance lifted Liriano's season ERA to 9.13.  In today's game of baseball, I would consider anything less than 4.00 to be pretty good.  Under 3.00 is excellent.