Charter School Studies: Apples to Apples vs. Apples to Oranges

Published November 1, 2004

The New York Times created consternation in the charter school community when it gave favorable front-page coverage to an American Federation of Teachers (AFT) study purporting to show charter students score lower on national assessments of reading and mathematics than do students in regular public schools.

The AFT study looked at a small sample, just 3 percent of charter students, gleaned from National Assessment of Educational Progress data, and failed to take into account the fact that charter schools serve a higher proportion of low-income urban students than do the affluent suburban schools to which the charters were compared.

After weeks of angry op-eds and letters to the editor from charter supporters, the strongest response to the Times/AFT report came in the form of a new study of charter schools by Harvard University economist Caroline M. Hoxby.

In contrast to the AFT study’s small sample, Hoxby’s study encompassed 99 percent of charter school elementary pupils. She compared their performance on state-required exams to the performance of students at the nearest public schools or, in some cases, at schools not quite as close geographically but closer in racial composition.

In other words, Hoxby compared the performance of charter school students with the performance of students at the schools the charter kids most likely would have attended if charters had not been an option. She called this an “apples to apples” comparison, as contrasted with the AFT’s “apples to oranges.”

Compared to students in the nearest public schools, Hoxby found, charter students were 4 percent more likely to be proficient in reading and 2 percent more likely to be math proficient. Compared to students in the closest public schools with a similar racial profile, charter students were 5 percent more likely to achieve proficiency in reading and 3 percent more likely to be proficient in math.

Charter students tended to perform at the highest levels in states or jurisdictions where the charter movement is most robust, such as Arizona, California, Colorado, and the District of Columbia. In Arizona, charter fourth-graders were 7 percent more likely to be proficient in reading and math than were students in the nearest regular public schools. In Colorado, the proficiency edge for charter students was 11 percent in reading and math; in California, it was 8 percent in reading and 3 percent in math.

In Washington, DC, more than 11 percent of public school students are enrolled in charter schools–a far higher proportion than any state. And in DC, the charter students’ proficiency advantage is huge: 35 percent or more in both reading and math.

Only in North Carolina was charter students’ proficiency significantly lower–4 percent lower–in both reading and math, compared to the nearby public schools.

Hoxby made no assertion that her study is, or should be, the final word. It demonstrates, she said, that “although it is too early to draw sweeping conclusions, the initial indications are that the average student attending a charter school has higher achievement than he or she otherwise would.

“These initial indications should give policy-makers the patience to wait for the results of studies that follow students who are randomly assigned to attend and not attend charter schools.”

Randomized studies are considered the “gold standard” of research. Because many charter schools have more applicants than they can accept, they usually hold lotteries, which makes it possible to conduct randomized scientific studies. The two groups of students are similar: both wanted to attend charter schools, but only one won the lottery. As in medical trials, comparison of the performance of the two groups allows researchers to make an estimate of the effect of the “treatment,” which in this case is attending a charter school.

Another important factor is the “value added”–how much do charter schools and comparable public schools help students progress year to year? Neither the Hoxby nor the AFT study was designed to reach a conclusion on that question.

Hoxby said multiple studies of gold-standard caliber are in the works. The question many charter supporters have is this: Will The New York Times report the story above the front-page fold if those studies show beyond doubt that students learn more when they are able to choose charter schools?

Robert Holland ([email protected]) is a senior fellow at the Lexington Institute, a think tank in Arlington, Virginia.

For more information …

The September 2004 study by Caroline M. Hoxby, “A Straightforward Comparison of Charter Schools and Regular Public Schools in the United States,” can be found online at