At the heart of the Chicago teachers strike, a historic rift that left 350,000 students in limbo for more than a week, was a question that school systems across the nation are confronting: How much should teachers be accountable for the performance of their students?

More rigorous evaluations of teacher performance, particularly those that tie teachers' ratings to student improvement on standardized tests, have been a centerpiece of the Obama administration's education reform plans. Nearly two-thirds of all states have made changes to their teacher evaluation policies since 2009, according to the National Council on Teacher Quality, and Illinois state law now requires districts to consider student growth measurements for a "significant portion" of a teacher's rating.

The Chicago Teachers Union ultimately struck a deal Tuesday night that will phase in student test scores as part of a teacher's evaluation, with scores eventually accounting for up to 30 percent of an evaluation within three years.

But at privately managed charter schools, another central prong of education reform efforts in Chicago and nationwide, those rigid standards for teacher evaluations don't apply. Because non-union charter schools aren't part of the contract between Chicago Public Schools and the teachers' union, they are free to create their own guidelines for evaluating teachers.

Some charter networks in Chicago have no formal written evaluations at all, giving individual principals the discretion to hold teachers accountable as they choose. Charter operators say they still rely heavily on achievement data ranging from standardized test scores to college enrollment rates to attendance, and some factor test scores in for as much as 70 percent of a teacher's assessment.

Some rely mostly on classroom observation and peer evaluation, while other networks offer bonuses to teachers that meet targets on test score improvement. But the decision to hire, fire and promote is left to the individual schools, and not bound by an overall district policy or a union contract that stipulates system-wide performance goals.

"You don't have the same protections in a charter school, so that's what changes the equation," said Joel Klein, the former chancellor of the New York City Department of Education, who now runs the education division at News Corp. "When you go to a charter school you don't have life tenure and the kind of job protection that you would get in a public school. It's a very different model, so I don't think it's a question of having different evaluation systems as much as the fact that management has to bargain the way it evaluates what the preconditions of termination are."

The more individualized approach to assessing talent is a central advantage, charter school supporters argue: freed from the constraints of district mandates and union rules governing tenure, charters can tailor curriculum in ways that best suit student advancement and improve teaching skills. But the data supporting this viewpoint is highly mixed, both in Chicago and nationwide.

Several national studies have shown wide variation in student outcomes at charter schools versus traditional public school counterparts: a 2009 Stanford University nationwide study found that half of all charter schools examined had results that were no different from traditional public schools, while 37 percent of charter schools had significantly worse learning results and 17 percent of charters offered superior educational opportunities. The same study found that Chicago's charter schools specifically offered significantly higher gains in student learning than in traditional public schools.

Yet there have been vast differences in student performance on standardized tests at charter schools in Chicago, according to state testing data released last year. Some elementary and high schools within the same charter networks had passing rates on standardized tests that were far above and below the city's average.

Chicago Teachers Union representatives argue that the dual push by Mayor Rahm Emanuel's administration to expand charter schools and test-based teacher evaluations –- goals that mirror nationwide policy set by U.S. Education Secretary Arne Duncan, formerly a Chicago schools chief -- raises questions about consistency in a system that is funded with public dollars, regardless of who manages the school.

"It's not as if charter schools have such great test scores," said Carol Caref, head of research for the Chicago Teachers Union. "I think it exposes that the corporate push for teacher evaluation is not really grounded in trying to make the schools better. Because if it were really about that, why would you exempt charter schools?"

Charter schools argue that by definition they are bound by an agreement laid out in their contract with the district, which can be revoked if schools don't comply. Yet critics often point to the fact that only a fraction of charter school contracts are ever revoked by state authorizing agencies.

In the 15 years since Chicago opened its first charter school, only five charters have been closed at the end of their contracts. Last year Chicago Public Schools announced a new initiative, funded by the Bill & Melinda Gates Foundation, to better align standards in charter contracts with requirements for traditional schools. There are now more than 110 charter campuses that serve 11 percent of all Chicago Public Schools students.

Nationwide, fewer and fewer percentages of charters are having contracts revoked each year: 6.2 percent of charter schools up for renewal were denied in the 2010-11 school year, down from 12.6 percent in 2008-09, according to a study from the National Association of Charter School Authorizers.

"The assumption has always been accountability for the school, in exchange for autonomy," said Heather Peske, a vice president at Teach Plus, an organization that works to improve teacher quality for urban students. "We've seen that bargain hasn't always played out very well."

Peske worked on a 2010 report published by the Center for American Progress that is one of the few to look at how some charter school management organizations approach the idea of teacher evaluation. One observation was that teaching in charter schools was "a very public act," with class sessions often videotaped for discussion and improvement, she said.

But when asked questions specifically about evaluations, many of the charter school teachers had a puzzled look.

"They were very confused by the term 'evaluation,'" said Morgaen Donaldson, who co-authored the report and is an assistant professor of education leadership at the University of Connecticut. "It was 'evaluation,' but it was really much more focused on growth and continuous improvement."

Chicago charter school operators all described different structures to their evaluation plans.

Michael Milkie, a former Chicago Public Schools Teacher who ventured into charters in 1999 and now runs the Noble Network of Charter Schools, said the evaluation at his high schools is done in an informal way, with no grades or official ratings. Each principal is given the authority to make decisions at the individual school level.

Rather than tying a teacher's tenure to standardized test scores, Noble's pay structure uses a bonus system to reward teachers -- part of which is measured by student improvement in test scores. The schools still collect data on individual teachers and measure various test scores at the beginning and end of a school year, but principals make the ultimate decisions on teacher retention and promotion.

"We provide them with all that information, and then leave it in their hands to make the best judgment," said Milkie, the co-founder and chief executive of the Noble Network. "Evaluation of teachers is a very tricky thing, and best done by educators at the school level, not by downtown districts or state legislatures."

The Perspectives Charter Schools system has a more formalized evaluation system that uses improved standardized test scores as 20 percent of a teacher's evaluation, and another 20 percent based on student improvement on other in-class assignments and tests. Other requirements include a minimum 2-point improvement on ACT scores from the beginning to the end of a school year, said Kim Day, the founder and chief education officer at Perspectives.

One of the primary reasons that evaluation systems at charter schools tend to be less strict is the far smaller size and scope of such schools, say education researchers.

"It's possible to have a much more individualized and humanized evaluation process than it is in a large district with a union contract where you know you're going to be sued if you don't have everything down in fine print," said Grover J. Whitehurst, director of the Brown Center on Education Policy at the Brookings Institution. "It's the difference between the type of feedback I give to my son when he's at home, and the kind of feedback he would get from the university he's attending."

Whereas principals at charter schools have frequent interactions with -- and control over -- the teachers working under them, school reformers argue that there was not a proper system to distinguish high- and low-performers in a massive district of more than 600 schools, like that of Chicago.

To get a sense, Chicago's prior evaluation system identified 93 percent of teachers as either superior or excellent, even though 66 percent of Chicago public schools were failing to meet state standards, according to a study by the University of Chicago's Consortium on Chicago School Research.

"The fact is that those test score gains are the best predictors we have available of how teachers are going to do in some subsequent year," Whitehurst said. "They're really imperfect, but what are the other two options that are available in a school district? One is 'everybody gets an A,' which has always been the status quo, or you just close your eyes and throw darts at the dartboard."

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