The National League sets itself apart from the American League in that the pitcher has to bat in the #9 spot. With the pitcher coming up to bat at least once every 2-3 innings, National League pitchers are at an extreme advantage because they are almost guaranteed an out.
As a result of the pitcher batting, many pitchers get out of jams easily or walk the #8 hitter to get to the pitcher. In the end, National League starting pitchers generally have an ERA that is about 1 full point better than their American League counterparts.
It is also a fact that the majority of National League ballparks are created to be pitching friendly. So even when a great pitcher in the National League has an off day, he can generally still have a great start because of the pitcher batting and his ability to walk the #8 hitter if he gets into jams.
As it pertains to betting, favorites in the National League play a lot better than favorites in the American League. Since the odds for baseball betting are almost exclusively created based on starting pitching, a dominant starting pitcher in the right situation against a team that he generally has success against will almost certainly win in the weak hitting National League.
As a general rule of thumb, I look for National League favorites and American League underdogs.
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