Private School Performance?

Infographic available at https://infogr.am/selected-schools-gpa/

In speaking with friends recently, we started puzzling over a question we all share: how do we know what the benefits are of a private school over public school? Clearly, many families see the benefit of substantial expenditures on private education, and surely there must be some return on that investment?  In essence, the consensus amongst the group was that the marketing of such schools leads parents to believe that the students that attend those schools would be expected, test scores and grades in high school being equal, to outperform public school students when the public and private school students arrive together at college.

In general, I have always thought that there is no way to determine the truth of that proposition -- that there is no useful data concerning performance in the first year of college.  Grades issued by different institutions are generally not comparable, even if they were available.  We tend to instead retask predictive measures of college success, such as the SAT, as a proxy for that data -- but such measures are always second best to the grades themselves.

But it looks like there's a way to answer the question after all.  The California State University system accepts students from virtually every high school in California (1,590 different ones in the last 18 years). While CSU is not the most elite part of California's Education Master Plan (that would be the UC, which accepts only the top 12.5% of high school students), CSU generally accepts the top third of California's students. While these students are not typically those who have accumulated many advanced placement units, and while these students tend to have test scores within one standard deviation of the norm, the students that attend CSU from private and public schools, at least as a matter of general consensus, are believed to have similar if not the same test scores and grades.  Thus, the performance of such students is perhaps the best example of head-to-head, apples-to-apples performance of public and private school students, after arriving at college, in California (if not in America).

I was surprised to discover last week that CSU makes those grades available on the web, sorted by high school and by year.  It was a relatively straightforward task to script cURL to pull CSU's data for 31 high schools in Marin, Napa, and Sonoma County from 1996-2012 (527 requests, which took the bash script less than 60 seconds), and I have zipped those files and made them available here, for anyone interested in examining the raw data. 

At the top of this post is a graph of the average freshman year college GPAs of students who have graduated from Sonoma Valley High School, Justin Siena, Sonoma Academy, Cardinal Newman, St. Vincent's, and Marin Catholic in that time period. While in 1996 every private school's college freshmen performed better than those from Sonoma Valley, the reverse has been true since 2005.  Sonoma Valley's students outperform every one of the private schools, and by a significant amount.

I've excluded Marin Academy, because they don't have enough data to graph.  Marin Academy sent six students to CSU in 2012, but before that Marin Academy fell below the threshold for reporting (5 students) every year -- so there's only one year's worth of data.  It's a good result for them, though -- in 2012, Marin Academy posted a 3.22, but their difference from Sonoma Valley (.17 of a grade point, 3.05 versus 3.22) is significantly smaller than the difference between Sonoma Valley and the next private school on the list (a .27 difference, between Sonoma Valley at 3.05 and Cardinal Newman, at 2.78).

Elizabeth Warren
Image available at http://tinyurl.com/ml4sw9g

This strongly suggests that those pursuing private education, with the exception of the very expensive Marin Academy (tuition $37,430 yearly), are perhaps subject to the phenomenon uncovered by Elizabeth Warren in her early research.  In "The Two Income Trap," Warren and her co-author (and daughter!) Amelia Tyagi pointed out that middle class families drive themselves into bankruptcy to buy homes they cannot afford in order to live in a neighborhood with better schools. As Warren and Tyagi argued, the actual "benefits" such parents obtained for their children were slim at best, and were more likely than not illusory in truth.

Yet the problem in Sonoma may be even worse.  Many parents are sending their children to private schools in the belief that they are obtaining an academic advantage.  This is not to discount other reasons for sending children, for instance, to religious schools -- educating one's children regarding deep religious convictions, shared by an organized group, and intimately related to daily living is a right protected by the First Amendment. However, to the extent that parents are financially straining themselves to obtain a perceived academic advantage, they should know that the evidence shows no increase in the children's later academic success, and instead shows that the opposite may in fact be true ...