As the Chicago teachers strike enters its fourth day, an issue at the heart of the conflict — how to fairly judge the performance of teachers — is playing out in classrooms and legislatures around the country.

Chicago isn’t the first place to see a clash between a national reform movement demanding accountability and labor unions trying to protect teachers from arbitrary firings. And it won’t be the last.

Since the District of Columbia first used student test scores to evaluate teacher performance in 2009, about 30 states have begun requiring school districts to rate teachers based in part on student scores.

The quick adoption was driven by Obama administration policies, with most states pledging to establish new teacher and principal evaluation systems in order to compete for money under President Obama’s $4.3 billion Race to the Top contest. States seeking waivers from the administration for some of the No Child Left Behind law’s requirements also had to adopt education policies that include judging teacher performance based in part on student test scores.

Lawmakers in Illinois, as part of the state’s bid under Race to the Top, passed legislation last year requiring that at least 25 percent of a teacher’s rating be based on student test scores.

In Chicago, Mayor Rahm Emanuel drew the ire of the Chicago Teachers Union in part because he wants to increase the weight given to student test scores over five years, with the scores ultimately accounting for 40 percent of a teacher’s job performance rating. Those with consistently poor ratings would lose their jobs.

Emanuel is not alone in placing a heavy value on student scores. In at least 13 states, test scores account for 50 percent of a teacher’s rating, according to the Education Commission of the States.

Still, measuring good teaching is complex, and unions and experts who have studied evaluation systems say student scores should not make up 50 percent of a teacher’s job rating.

“Some states are now considering plans that would give as much as 50 percent of the weight in teacher evaluation and compensation decisions to scores on existing tests of basic skills in math and reading,” 10 leading academics wrote in a 2010 paper published by the Economic Policy Institute. “Based on the evidence, we consider this unwise. Any sound evaluation will necessarily involve a balancing of many factors that provide a more accurate view of what teachers in fact do in the classroom.”

Critics say the emphasis on test scores places too much pressure on teachers and students, creating a “teaching to the test” mind-set, narrowing the curriculum and, in the worst cases, resulting in cheating scandals.

In nearly every place where teacher evaluation systems have been installed, education officials regularly tweak them.

“It is very, very difficult to get right, and if you don’t get it right it’s going to blow up,” said Kathy Christie, a vice president at the nonpartisan Education Commission of the States.

After months of discussions, Missouri lawmakers adjourned for the summer rather than try to come to agreement on their teacher evaluation legislation. In Florida, the teachers union is suing the state over its new evaluation plan.

In the District, school officials last month dialed back the emphasis on student scores on standardized tests from 50 percent of a teacher’s rating to 35 percent.

With the changes, the other 15 percent of a teacher’s rating is still based on student performance but not standardized test scores — the teacher and principal decide the right gauge to use, according to Melissa Salmanowitz, a spokeswoman for the school system.

“We wanted to reduce some of the anxiety that even our best teachers feel” about the use of standardized test scores, she said in an e-mail.