Here's a fun tool to calculate win probability added, from Hardball Times.
http://www.hardballtimes.com/thtstats/other/wpa_inquirer.phpOne thing you'll need to input is run environment, or average runs. A way to do that is to go to ESPN's stat site, look at MLB runs, calculate an average, take the park factor (also on ESPN) , and multiply the park factor times the average runs. For example, Nats Park is playing as an slightly above average park for run production (1.025) and MLB average runs is 4.35, so the run environment in Nats park is about 4.5, to the nearest half run.
Suppose Burnett pitches a clean 8th with a 2 run lead. the difference in the likelihood of a win from bases empty top of the 8th with a 2 run lead and bases empty top of the 9th 2 run lead is 6% in a 4.5 run environment. the likelihood of win goes from 87% to 93%.
