What treatment does, and the honest state of the evidence
Two reviews, and neither is quoted with a number attached, because no effect size was read off either paper for this course. What follows is the direction of the findings and their limits.
What the research says
Del Vecchio T, O'Leary KD (2004). Effectiveness of anger treatments for specific anger problems: a meta-analytic review. Clinical Psychology Review, 24(1), 15 to 34 doi
Pulls together trials of anger treatment and asks whether they work differently for different kinds of anger problem. The useful conclusion for a reader is that anger is not one thing, and that the treatment has to be matched to the particular problem rather than picked off a shelf. No effect size is quoted here because none was read off the paper.
Henwood KS, Chou S, Browne KD (2015). A systematic review and meta-analysis on the effectiveness of CBT informed anger management. Aggression and Violent Behavior, 25, 280 to 292 doi
Reviews anger management built on cognitive behavioural methods and finds it helps. Its own limit is that most of the studies were run in forensic and offender settings, which is a strength because the outcomes are real behaviour, and a limit because those samples are not the general public. No effect size is quoted here because none was read off the paper.