The effects of temperature and ballparks still isn't a matter of opinion. They're derived from differences in scoring and batted ball results over large samples. They're not perfect, but treating raw slash lines from a Marlins player and a Rockies player the exact same is far less useful and more flawed. The thing about ERA though is that it is in fact subjective; it's subject to the judgement of the official scorer. An error can mean that a barrage of hits and walks after that are treated as unearned runs. Which is fair in the sense that those plate appearances shouldn't have happened, but as a measure of performance it's not very useful because they did happen. A different scorer (Home vs away, or if the batter has a hitting streak going or w/e) can judge plays differently. But in addition to subjectiveness, it's also lacking. ERA also does a poor job when it comes to relievers and inherited runners. Reliever inherits a runner at 1st with 2 outs. He proceeds to walk three straight batters before recording the final out. Starter gets docked for 1 ER, reliever for 0, which hardly reflects their performance that inning. It shouldn't be dismissed, but it's very important to not treat it as gospel, especially over small sample sizes. Because it is subjective, and in the short term can be very volatile and not really be reflective of performance, and even less so of talent. WHIP is better in some ways, and it is more objective, but it's very limited. It doesn't say anything about whether the hits were XBH or not. Nor does it take into account, for example, strikeouts or double plays; pitchers who generate a lot of both, or either, don't get punished as much for allowing baseunners. And over small samples, even hits allowed carries a lot of randomness and luck. The reason I like DRA better than some other metrics is that it's based on what happens at the plate. At its heart it's essentially the inverse of wOBA/wRC (And thus very similar in results to OPS), i.e it adds up all the outcomes of plate appearances; 1B, HR, BB, HBP, GIDP etc and their effect on run scoring. Using DRA (or wOBA/OPS against, though they don't include controlling the running game, or double plays) to judge pitching performance is similar to using wOBA/wRC+/OPS (i.e essentially the triple slash) to judge a players offense. Whereas ERA (or RA9) is like using RBI + runs to judge a players offense. The latter does tell you something, it tells you the eventual result of the players performance once his teammates performance is added onto it. And over the long run, they will line up pretty well with things like OPS (+ baserunning). But runs scored and runs allowed are still a product of singles and doubles and HRs and walks. So in looking to, in any way shape or form, try to evaluate which player will do better in the future it's better to look at stats based of inputs rather than results. People vastly underestimate the variance in baseball, and the effects of sequencing, that in turn affects run scoring. And just how big a sample you need to draw any conclusions from things like ERA. Anyway, regardless of ones view of what's relevant, what's objective or not, the methodology behind it etc, I'd like to return to my original point: To predict the future run prevention of this (or any) pitching staff, DRA is a much, much better metric than one month of ERA. Or two. Or three. That much is fact. For that matter, so are SIERA, xFIP, FIP and K%-BB%. Haven't seen any data on it, but I'd imagine that even WHIP beats ERA at this point. Use ERA for awards season, or for hall of fame voting, or to evaluate large chunks of time. But not for this.