The 2011 Official Playing Rules and Casebook of the National Football League devotes 1,127 words to the section on roughing the passer.
Twenty-two words near the end sum up the spirit:
"If in doubt about a roughness call or potentially dangerous tactic on the quarterback, the referee should always call roughing the passer."
That sentence pretty much absolves referees from blame for penalizing acts that seem to be borderline infractions.
We discussed one such penalty against the Arizona Cardinals' Calais Campbell earlier.
Campbell leads the NFC West in roughing-the-passer penalties since 2009 with three. Teammate Clark Haggans, the San Francisco 49ers' Ahmad Brooks, the Seattle Seahawks' Raheem Brock and ex-Seahawk Patrick Kerney have two apiece since then.
Instead of focusing on players, I've put together a chart showing how many roughing calls each of the 17 current referees has called since 2009. Note that Clete Blakeman was not a referee until 2010. Officiating crews change members from time to time, but the referees are the ones responsible for most roughing calls, so these numbers hold up better.
Some referees call more penalties than others overall. Some have surely encountered more instances of roughing than others. But if you're a defensive end eager to mete out some old-school punishment on the opposing quarterback, it wouldn't hurt to know which referee was working the game that day.