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"Fire the bum!" You don't have to spend much time in the stands of a football stadium, or bellied up at a sports bar, to hear that sentiment. (Probably in more colorful, modern vernacular, but you get the point.)

On cue, impatient NFL owners often comply. They have replaced 22 percent of their head coaches in each of the past two offseasons, and these days it is not unusual to see a disappointed team turn over a third of its roster from one season to the next. Far less visible, at least until quite recently, is a similar trend within the league's officiating program.

The NFL has replaced at least 20 of its officials in the past two offseasons, according to vice president of officiating Dean Blandino, and that number could reach 21 when the personnel process ends later this spring. On a roster of 119 officials league-wide, that represents a 17 percent turnover rate -- the highest over a two-year period in more than a decade. The NFL did not confirm officiating changes until Blandino ascended to his role in 2013. According to the web site FootballZebras.com, which independently tracks NFL officiating, the last significant personnel turnover ended in 2002.

A handful of the changes have coincided with voluntary retirements, but for the most part, the turnover is a result of an evaluation process that has intensified to match the rising external scrutiny of officiating decisions. Replay technology, and the proliferation of ex-officials providing instant broadcast analysis, has exposed officiating inefficiences that once went unnoticed. Blandino made clear recently that the league is taking overt steps to apply appropriate pressure on performance and develop a deep pool of replacements.

"We take it very seriously and we want the best officials," Blandino said. "We are going to do a comprehensive review of the entire staff every year and if an official isn't performing up to the standard, they won't be a part of the NFL. And we have many qualified candidates that are ready to step into that place. So the pressure is immense and we understand that responsibility."

Blandino hired 13 new officials after the 2013 season, including three new referees, and another nine officials this spring. No new referees have been announced for 2015, but the hiring of line judge Sarah Thomas -- the first female full-time official in the NFL -- brought national attention to the turnover. (The league is expected to have 120 or 121 officials on its roster this season to provide "swing depth" for injuries, vacations and other absences.)

As Blandino explained, the NFL's officiating department performs comprehensive postseason reviews of each member. They are assessed, among other things, on speed of decision-making, accuracy of calls and physical fitness -- the latter factor as important as any level of rules expertise in the era of spread offenses.

Each official is ranked annually in one of three tiers; the third is a grouping of the lowest performers. That collection receives what Blandino referred to as "enhanced training" and "a focus on them and their development." If they remain in the third tier the following year, they are candidates to be fired.

Replacements are culled from a group of 2,000 college officials who are part of the NFL's officiating development program, run by league scouts who observe about 1,200 high school and college games across the country. Of that group, 21 qualify for an advanced program that puts them one step away from an NFL call-up. In essence, the league has a Triple-A team of officials available each year to pressure incumbents.

Though there were still some high-profile calls in 2014, and in the end the analysis is subjective, Blandino said he was confident that officiating improved in the big picture last season. Was that a result of infusing more talented officials and putting other incumbents on notice? Perhaps. "Different" isn't always "better." But what's clear is that a longstanding good-old-boy network has been shaken up a bit, which is never a bad thing.

"We saw that officiating was better, but not just because of the turnover," Blandino said. "It's an overall difference in terms of what we are doing from a training perspective and ... the physical fitness perspective and what we are requiring of our game officials because the scrutiny is so high at this point. We really are asking more of them. Part of it was the turnover and the fact that we brought in 13 new officials and they performed well. Because that can go the other way.

"It's really, just like a team, finding the right balance of youth and experience and having it all mesh together. Obviously it's not perfect. We had challenges and bumps in the road. But we felt like getting the right people was key."