The following is an excerpt from the new book Beyond Test Scores: A Better Way to Measure School Quality by Jack Schneider (Harvard University Press, August 2017), available from Harvard University Press, Amazon, and IndieBound. Copyright © 2017 by the President and Fellows of Harvard College. Used by permission. All rights reserved:

Most people imagine themselves to be making informed decisions about their children’s futures, at least among those with the privilege to decide. Yet they often end up making decisions that reflect the limited nature of their information. Why is that? To get a better handle on the issue, it is useful to look at the examples of charter and private schools.

According to a 2012 Gallup poll, 78 percent of Americans believe that private schools provide a good or excellent education, and 60 percent believe the same about charter schools. Traditional public schools, by contrast, elicit that response from only 37 percent of the populace.

How accurate is the picture Americans have of private and charter schools? It’s a hard question to answer, not only because running a true experiment—taking a large number of children and randomly assigning them to different kinds of schools—is impossible, but also because school quality is such a multifaceted concept. Many scholars have tried to work around this problem, using pseudo-experiments and statistical techniques to measure traditional public schools against their counterparts in the private and charter sectors. But findings are anything but consistent, and it cannot be universally claimed that one kind of school is superior to the others. Just about the only conclusion we can safely draw is that some schools in each sector—whether private, charter, or traditional public—will be of high quality; others will not.

How can that be? Private schools send almost all of their students on to college—a substantially higher percentage than traditional public schools. Just ask their college guidance counselors; or simpler yet, look at their websites. Groton, for instance, has a web page listing colleges where five or more of its students have matriculated over the past ve years. On that list are schools such as Harvard, Yale, Dartmouth, Princeton, Columbia, Brown, and Penn. Fact sheets like this can be quite persuasive, as can the advertisements known as “look books” sent to prospective families. At one New England private school, the 2016–17 look book—a seventy-five page glossy magazine, which cost the school six dollars just to mail—closed with a list of colleges attended by graduates, as well as the colleges attended by faculty.

Yet it is quite possible that the admissions edge private school students seem to have is not due to schooling. Roughly 80 percent of students from high-income families, regardless of the kind of school they attend, immediately enroll in college after high school; that is true for only about half of students from low-income families. Among households with incomes under $50,000, only 6 percent attend private schools. By contrast, 26 percent of households with annual incomes of $200,000 or more send their children to private schools. Thus, we might reason that when we look at private schools, we are really just seeing the effects of concentrated privilege.

This is not to say that there are no differences between private schools and public schools, especially when the private schools in question are elite institutions with massive endowments. But many of the differences between private schools and public schools are magnified out of proportion. Parents often believe, for example, that private schools offer much safer environments for their children, or that class sizes in private schools are significantly smaller. According to the National Center for Education Statistics, however, crime rates and incidents of bullying are roughly the same across all kinds of schools, and average class sizes are nearly indistinguishable at both the elementary (20.3 public, 18.1 private) and high school (18.6 public, 18.4 private) levels.

Charter schools, too, maintain solid reputations among the American public. As with private schools, though, this may not be due to any real difference in quality. Like their private counterparts, charters have self-selecting populations. This can be due to pull factors, such as well-crafted messages sent to would-be parents, or to push factors, such as strict rules and high academic expectations. In either case, however, it complicates what we can say about the value added by a charter school. And though thoughtful researchers have tried to account for this by using a variety of methodological approaches and statistical techniques, it can be very hard to pinpoint which results are due to the student’s school and which to the student’s family. Thus, the only generalizable thing we can say about charters is that quality varies from state to state and from school to school. Some cities have stronger charter sectors than others, and some charter firms have more coherent practices than others.