Not very accurate at all. These numbers were calculated by taking the average salary of physicians of the given field and assuming a 40 hour work week as the first comment on the article points out. I don't have solid data, but take OB/Gyn vs. psych:
According to Medscape's physician compensation report, OB/Gyn averaged 286K while psych averaged 235k.
Psych legitimately averages around a 40 hour work week, and the reports I've seen say it's actually closer to 35, but we'll stick with 40. That means someone working 48 weeks of the year at 40 hours per week is at 1920 hours for the year, so 235,000/1920 = $122.40/hr.
The numbers I've seen for OB/Gyn say they're working on average close to 60 hours/wk, but we'll say 55 (which I think most fields will be close to). For 48 weeks at 55 hours per week, that's 2,640 hours/yr. 285,000/2640 = 107.95/hr, $15/hr less than psychiatry.
So according to Becker's, OB/Gyn is making $40+/hr more than psych, which I have a hard time believing given the calculations I just did and considering how much worse the average OB's work week is compared to an average psychiatrist. I don't think there is a truly accurate report or metric that has been put out for hourly wage, or if there is I'm not aware of it. I think in general Derm is king in terms of hourly wage along with procedural subspecialties (ortho/spine and neurosurg) up there as well. There are also certain models that can pull in bank like cash only PP or strong DPC markets, but those are far less common.