Two recent studies of student performance in charter public schools have rekindled the great debate over the expansion of charter schooling. Meanwhile, the U.S. Department of Education's "Race to the Top" program fans the flames by promising millions of dollars to states that become more charter-friendly. Missing from this debate on the effectiveness of charter schools, however, is one crucial piece of evidence pointing to their success: parents want more of them.

Although they use different methodology, both studies compare charter school student achievement with that of conventional schools. In June, the Center for Research on Education Outcomes released its report showing that 17 percent of charter schools outperformed conventional schools in math. Nearly half of the charter schools were on par and 37 percent performed worse than conventional schools.

A few months later, Caroline Hoxby of Stanford published a report with very different results. Controlling for a common charter school criticism, selection bias, Hoxby found New York City charter schools significantly increased student achievement. The study's most significant finding was the rapid closing of achievement gaps between urban students in charters and suburban students in conventional schools.

Each study has its critics, and it's reasonable to assume that no study will perfectly measure the effectiveness of charter schools at bolstering student achievement. The fact remains that many individual variables go into determining student achievement. While student achievement shouldn't be completely abandoned as a measure of school success, it also shouldn't be the end-all, be-all.

Measuring quality in a monopolized system is always difficult, and conventional schools have operated as such for decades. The advent of charter schools and other forms of school choice ushered in a new element -competition- that created a new measure of quality. Some parents are now allowed to choose their children's school, and this creates differing levels of demand for different types of schools. The varying levels of demand should contribute to measuring school effectiveness; if parents want more of them, on some level, the schools are succeeding.

Policymakers should consider recent trends in parental demand. The figures are telling. On a national scale, the percentage of students not enrolled in their address-assigned school increased from 20 percent to 27 percent from 1993 to 2007, according to the National Center for Education Statistics. Similarly, the Michigan Senate Fiscal Agency reports that the number of Michigan students not enrolled in their state-assigned school increased by 70 percent from 2002 to 2008.

As should be expected, the decreasing demand for state-assigned conventional schools corresponds with an increased demand for charter schools. There are currently about 1.5 million student enrolled nationally in 5,000 charter schools around the country. However, there could be many more. It's estimated that 365,000 students are on charter school waiting lists. According to the Michigan Association of Public School Academies, there are at least 12,000 students on waiting lists in Michigan.

So why do states like Michigan limit the growth of charter schools if large waiting lists exist?  The unfortunate answer is that the wishes of parents are not factored into the charter school debate. Typically, the debate is limited to the success of charter schools in relation to conventional schools as judged by studies like the two mentioned above. Most of the discussion in this debate revolves around the methodology and applicability of these studies, and the demands of parents are subsequently ignored.

For the most part, this plays into the hands of the opponents of charter school expansion, school choice and the defenders of the status quo. For every study that shows improved student achievement in charters, charter opponents can point to another study that muddies the waters and shows mixed results.

No matter how successful they are at muddying the water on charter school achievement, those opposed to more charters in Michigan must face the fact that the parents of 12,000 students demand a different educational opportunity for their children than the one proscribed to them by the state.

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Michael Van Beek is director of education policy at the Mackinac Center for Public Policy, a research and educational institute headquartered in Midland, Mich. Permission to reprint in whole or in part is hereby granted, provided that the author and the Center are properly cited.