Some of the data used to generate this chart is derived from Baseball Musings' Day-by-day database. Hitter ratings account for the opposing starting pitcher's past history (three years' worth as well as past 21 days) as well as ballpark factors. 'LHB' and 'RHB' ratings account only for left- and right-handed batters, respectively. OPS is the primary statistic used in the calculation. Ratings range from 1-10, with 10 representing the best possible matchup, statistically speaking, and 1 representing the worst. So, for example, a 10 is a must-start rating, while a 1 should be avoided (if possible); a 1-2 is poor, 3-4 is fair, 5-6 is average, 7-8 is very good and 9-10 is excellent.