Twenty years ago, the Pennsylvania charter school law was signed. Since then, charter schools have been heralded as the savior of the public education system.

Have charter schools improved educational attainment and lowered costs, as so many claim? The answer to those questions and just about every other concerning the impact of charter schools is simple: We have absolutely no idea.  And that's a big problem.

So, what are charter schools? They are publicly funded, privately run schools. Their revenues come out of the budgets of their students' school districts through a state-mandated funding formula. The amount per student is dependent on the funding level of those districts.

Charter schools have more freedom in hiring and spending money. The intention was to allow smaller schools to experiment with different ways to educate students and/or concentrate on a specialty, such as science, math, or arts. That was supposed to improve educational outcomes.

The charter school movement, at least when it comes to enrollment, has been extremely successful. In Philadelphia alone, there are 84 brick-and-mortar charters educating about 70,000 students. About 30 percent of the school district's budget goes to charter schools.

As for educational performance, that's a different story. Put simply, there is absolutely no study that shows charter schools have improved student outcomes in Philadelphia.

It's not that I haven't tried to find one. I searched the literature and the internet. I called local charter school organizations asking if they were familiar with any study that compared charter school performance with public school performance in Philadelphia, the suburbs or Pennsylvania. The answer: There was none that they were aware of.

I'm not saying charter schools haven't improved educational performance.  It's just that there is no research that shows they have.

What about lowering the cost of education? Again, there are no studies that provide the data that prove charter schools are run more cost-effectively. Indeed, one of the biggest problems with the charter school law is that school finances are largely opaque.

Search for information on how charter schools spend their funding, and you get few details.  What are their teacher pay schedules? Got me. How much do schools spend on technology, and what kind of technology? No idea. What is management compensation, or the return to investors? Who knows?  Ask for the same information from the public schools, and you get answers.

Again, I am not saying charter schools don't spend their funds effectively. It's just that we have absolutely no idea if they do because of the lack of financial transparency. Even the well-known Stanford University national charter school study contained no financial analysis.  It was as if the amount of money and the way it is spent were irrelevant to educational outcomes.

The structure of the Pennsylvania funding formula, written specifically to provide financial support for charter schools, is one of the Philadelphia School District's biggest problems.  It makes little difference that large school districts have a much wider set of needs, some of them extraordinary, than a typical charter school, and especially cyber-schools. One size fits all when it comes to charter school funding.

An example of the funding-formula problems could be seen in the Philadelphia district's negotiations with the teachers union. The district worried that a teacher pay hike would trigger a funding increase for charter schools, raising the total cost of the agreement. That would happen even if the charter schools had no change in their cost structures. Meanwhile, the charter schools could spend the windfall any way they choose, be it on teachers, school supplies, technology, managers, or even investors.

There is one thing we can clearly say about charter schools: Parents are generally happy with them. However, we don't know why. The levels of satisfaction were high even for schools closed due to poor performance.

Charter schools are the largest educational experiment since public education became mandatory, and after 20 years, we should know the impact they have had on educational performance.

We should know if better ways to organize educational activities have been discovered. We should have learned whether there are more effective ways to spend money.  We should know if specialized schools actually work. Unfortunately, we know none of that.

The time is now for the Pennsylvania Department of Education to fund a major research project that determines the impact on educational performance of charter schools before we rush headlong into spending billions of dollars more and changing the fundamental structure of public education.