When Kevin Strawn's principal showed him a new Charlotte-Mecklenburg Schools rating of his effectiveness, he was stunned.

Based on the number, the East Meck math teacher had done less to help his students master Algebra II than most of his colleagues. His immediate question: What am I doing wrong?

"I am Mr. Lower Quartile," Strawn recently told the school board, using the CMS label for its least-effective teachers. "I have no idea why, and no one's been able to tell me why."

About 40 percent of CMS instructors - those who teach classes with state exams - have been rated using a new "value-added" formula. It's designed to tease out the part of each child's academic progress that's caused by the teacher.

Those ratings anchor Superintendent Peter Gorman's quest to figure out which teachers are achieving results and to tie their pay to their skill with students. CMS teachers, who must approve any such plan, worry that the change may instead drive top teachers away.

Parents can't see the ratings, which were presented to faculty last summer. Gorman says they're part of teachers' confidential evaluations.

At the Observer's request, CMS has released a compilation of the new ratings for each school. Those numbers raise questions about who's getting the district's best and worst teachers, based on the new measure. They're likely to rattle parents' perceptions.

For instance: Wilson Middle, a long-struggling high-poverty school, is rated near the top for teacher effectiveness.

By the same measure, Davidson IB, an academic powerhouse, drags the bottom.

West Charlotte and West Mecklenburg highs have more top-rated teachers than Providence, Myers Park or Ardrey Kell.

"What we've seen is, we have superstars at every school, including our schools that have historically been low-performing," said Susan Norwood, executive director of a CMS performance-pay pilot.

The value of a teacher

Value-added ratings are the latest twist in the national push to use student test scores to judge teachers. Proficiency rates and year-to-year student gains are already used to rate N.C. schools.

But much of what shapes academic success lies outside the classroom. Does the child have a stable home, full of books, conversation and opportunities to learn? Is he homeless, sick or hungry? Do her parents speak English?

Value-added ratings attempt to mathematically account for those variables. They've been around for years, used in academic research and in other school districts.

CMS is devising its own formula. Starting with students' previous test scores, it projects how they should do the next year, looking at such things as homelessness, disabilities and fluency in English.

Absences and disciplinary actions are also factored in, as a gauge of factors that can hinder learning no matter how good the teacher.

If a student performs better than predicted, that's considered the teacher's added value.

Last summer, CMS rolled out its preliminary results for the first time, showing teachers how they compared with colleagues across the district.

Officials say value-added ratings will never be a perfect gauge of teaching. For that reason, they will be one of several measures used to determine pay. They're working with faculty, principals and others to craft an array of measures to use in performance pay, from classroom observations to learning goals teachers design for their students.

But the ratings form the foundation of CMS thinking. For instance, when Gorman says experience and credentials don't reliably translate to effectiveness, he's basing that on value-added scores.

"If we don't talk about differences in effectiveness of teachers," Gorman told the school board in December, "we're jeopardizing our students."

How to use them

The new ratings don't affect pay yet, except at 20 CMS schools involved in a performance-pay pilot. And Gorman says they won't determine who gets laid off in the expected 2011 budget cuts.

But they're already shaping decisions about teaching.

At Davidson IB, the low ratings were a shock.

The school attracts students already performing at high levels. Its International Baccalaureate program emphasizes foreign language, high-level thinking and classes that go beyond basic reading and math.

After studying the 2010 ratings, Principal Jo Karney realized her students were spending less time on math and language arts than those at many other middle schools. Knowing how high the stakes will eventually be for her teachers, she juggled this year's schedule, cutting into social studies and science to provide more time on subjects with state exams.

No one knows whether that will benefit the students.

"It's an imperfect tool at this point," Karney says.

Andy Baxter, CMS' performance-pay director, agrees. He's trying to figure out whether the value-added list accurately reflects the performance of IB students. But he says the numbers provide valuable data that will eventually help more kids get better teachers.

Others are skeptical of the whole premise.

Such ratings are only as good as the tests they're based on - and that, some critics say, makes them nearly worthless. Multiple-choice tests of the sort North Carolina uses are relatively cheap to give and grade, but many say they reveal little about the kind of learning that boosts adult success.

About 60 percent of CMS teachers don't have value-added ratings because their kids don't take state exams. CMS is designing new tests, mostly multiple-choice, to help rate those instructors.

Meanwhile, there's still debate about how well value-added ratings identify truly successful teachers.

A handful of principals told the Observer the ratings match their own views of who's doing the best job.

But a middle-school teacher, who asked not to be identified for fear of reprisal, voiced the opposite. He said he heard a presentation on how his school fared on each subject. The school is small enough, he said, that it was easy to match ratings with individual teachers.

The school's well-regarded algebra teacher, he said, has seen test scores rise over the years. But CMS' ratings showed algebra effectiveness as low-average and declining.

"It seems to fly in the face of what we know to be true in the classroom," the teacher said.

Individual ratings

Perhaps the most explosive issue on the national scene is the release of individual teacher ratings, which has spurred protests and court battles.

People who support the release say that if schools can calculate a teacher's impact and hinge public spending on such ratings, parents/taxpayers should see the numbers.

Opponents say such data reduce a career to one number, no matter the disclaimers attached. Many teachers say the release can create a virtual blacklist as parents clamor to get their children assigned to high-rated instructors.

N.C. law exempts portions of personnel files from public inspection, and Gorman considers the value-added rating part of the protected information.

Strawn, who became a teacher in 2000 after a long career as a computer programmer, chose to make his own ratings public during a school board discussion. "I know what my kids' problems are and what's holding them back," Strawn said after speaking to the board. "I don't know how to help them get over those obstacles."

Gorman praised his candor and said Strawn's frustration is legitimate.

For now, a value-added rating provides no insight on how to get better. But CMS is also working with university researchers and volunteer teachers, trying to identify classroom techniques that work so they can be shared.

Competition vs. teamwork

Many teachers say percentile rankings erode the teamwork that can help a school improve. If teachers compete for top ranking and pay, the argument goes, there's little incentive to help colleagues.

Baxter says one way to offset that is to consider schoolwide and/or team performance as part of the final rating. And he and Gorman say the value-added numbers will never be the only measure of good teaching. Groups of teachers are studying other criteria, including how to size up a teacher's contributions to colleagues and the school.

Mary Sturge, principal of Reid Park Elementary, is part of the CMS pilot program that awards bonuses based partly on value-added ratings. She says that motivates teachers to challenge colleagues who may not be giving their best.

"The stakes are so high right now that ... a crackerjack teacher is not going to tolerate a laissez faire one."