If a staff hurler makes it to the big leagues they have demonstrated a high level of skill and produced quantifiable statistics such as ERA, wins, strikeouts, quality starts or FIP. But which commonly accepted metric designates them as an elite or ace pitcher?
closed as unclear what you're asking by Azik Abdullah, wax eagle, Nick, user527 Jun 10 '14 at 14:32
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There are many different ways to define an "ace". He is the best starting pitcher on a pitching staff. "Ace" being a common nickname for the number 1. A team's number 1 starter is often called their ace even if he isn't as dominant as other pitchers. A player that wears the number 1 may be referred to as "Ace" as a nickname regardless of his position.
The term ace is also referred to as a dominant pitcher in the league period. Where he may not be the best pitcher on his staff, but on most others he would be the number 1. A good example of this is the LA Dodgers pitching staff. Zack Greinke and Clayton Kershaw are both seen as aces on the same staff, even though Kershaw was the number 1 starter to being the season and Greinke was the number 2.
As far as seeing a pitcher as dominant, it changes from person to person and team to team. Some teams will value pitchers that throw a lot of innings (200+). Some will value a guy that has many quality starts. With the addition of sabermetrics there are many other ways to quantify a pitcher's value.
The only other thing I'd add is that baseball is very developmental. There is a large minor league system for development. Many pitchers may be valued at a young age because of the potential, but don't end up panning out. Some pitchers may become an ace for a short period of time due to age, injury, or any other factors.