Denver Public Schools’ decade-old pay-for-performance system doesn’t always result in bigger paychecks for the best teachers — and its impact on student achievement is mixed.

Those findings are included in a new report by the pro-reform education advocacy group A Plus Colorado. It recommends that as the school district and teachers union sit down to renegotiate the terms of the pay system, they consider revisions that include more closely tying teacher evaluations to compensation.

The report recounts the history of DPS’s system, known as ProComp, and summarizes research on whether it’s working as intended.

The short answer? It’s not entirely clear. The report quotes an early backer of ProComp as saying the purpose was to pay teachers “for getting results with their kids.”

But as the report points out, several studies have failed to uncover a direct link. One found that teachers whose students showed the highest academic growth in math earned similar salaries to those whose students showed the lowest growth in math.

ProComp is also supposed to incentivize teachers to work in the most challenging schools. But the report says research has revealed the extra money offered for working in “hard-to-serve” schools hasn’t significantly reduced turnover in those schools.

And it notes that while teachers earn more under ProComp than they would under a traditional salary schedule, studies have come to different conclusions about how much.

Plus, as we’ve reported, teachers find ProComp confusing and sometimes unfair.

A Plus Colorado offers several ideas for how to overhaul the system to solve several problems facing the district, including new teachers leaving DPS after a few years and attracting teachers to work in high-needs schools. They include:

— Offer the biggest raises in the first five years of a teacher’s career to increase retention.

— Simplify the salary schedule to offer two “lanes” based on teachers’ level of education instead of seven. Under that schedule, subject matter experts would earn more than general teachers.

— Increase the amount of money offered to teachers for working in hard-to-serve schools, and pay teachers even more for returning to teach at the same high-needs school.

— Instead of offering stipends to teachers who take on leadership roles, reward them with increases to their base salary that go up as they take on more responsibility.

— Make teacher evaluations “a primary way to accelerate earnings,” but only after the district strengthens and clarifies its evaluation system. For example, a teacher who earns a “distinguished” rating could move up a step on the salary ladder.

The current ProComp agreement expires on Dec. 31, 2017.

The district and the union have not started negotiating. But they are talking to each other in what the union’s executive director called an “exploratory phase.” Among the questions they’re trying to answer: What should the goals of a pay-for-performance system be?

Read the entire A Plus Colorado report below.