(Reuters) - A unique, incentive-based pay structure adopted by Denver public schools more than a decade ago is at the crux of a strike by teachers who say the bonus system has eroded their earning power in a city where the cost of living has soared over the last 10 years.

Munroe Elementary teacher Melissa Curry holds a sign during a rally across from the Colorado State Capitol as Denver public school teachers strike for a second day in Denver, Colorado, U.S., February 12, 2019. REUTERS/Michael Ciaglo

Their union, the Denver Classroom Teachers Association, initially embraced the so-called ProComp pay scheme but is now seeking a more traditional salary structure with less emphasis on bonuses tied to student achievement or tougher teaching assignments.

The Denver work stoppage, which began on Monday, follows statewide teacher walkouts driven by salary disputes last year in West Virginia, Kentucky, Oklahoma and Arizona, and a strike in Los Angeles last month that focused on pay, class size and charter school regulation.

The strike in Denver is contesting the longest-running teacher compensation system of its kind in the United States, according to Allison Atteberry, an education professor at Colorado University-Boulder who has studied the issue.

ProComp, short for Professional Compensation, began on a pilot basis in 2001, growing out of a national movement to link teachers’ salaries with performance, measured in part by student achievement.

The Denver program included incentives tied to performance evaluations, and a panoply of other factors aimed at getting the strongest educators to the students who need them most.

MINIMIZED BASE PAY

As fully adopted in 2005 - with a special voter-approved property tax to fund it - ProComp includes bonuses for teaching in high-poverty communities and in hard-to-staff subjects such as math, science and special education. Conversely, it also rewards faculty members of the top-performing schools.

But union officials said the program was revised in 2008 in a way that has minimized general base pay in lieu of bonuses that are less predictable and have failed to keep pace with rising living expenses.

The result is a growing exodus of experienced teachers from Denver to neighboring districts with higher pay, said Robert Gould, chief negotiator for the 5,650-member Denver Classroom Teachers Association.

The cost of a median-price home in Denver has jumped 85 percent during the past decade, while Colorado as a whole ranked 50th last year among all states in teacher wage competitiveness in a Rutgers University study.

“This is really at the heart of what Denver teachers are experiencing; an unlivable overall salary level in an increasingly unaffordable city,” Atteberry told Reuters by email.

The superintendent of Denver Public Schools, Susana Cordova, indicated at the outset of renewed contract talks on Tuesday the district was moving toward teachers’ demands for simplifying the current pay structure.

“Many of the things I think we hear our teachers complain about, actually, aren’t’ about the proposal that we’ve put on the table,” she said. “It’s about the current system. And many of those things I agree with as well.”