Burying the Lead: Grade Inflation and the Public-Private Divide

The following graph (available here at Flowing Data) has been doing the blog and twitter circuit the past few days. The data, collected by Stuart Rojstaczer, shows the average GPA for public (blue line) and private (green line) schools from the 1920s to today.  What we see, immediately, is a rapid rise in grade point averages beginning in the 1960s.  John Sides sums it up nicely: “Yikes yikes yikes.”
What I was surprised by, and what I didn’t see mentioned in the other accounts of this trend, is the amazing disparity between public and private schools.  From the 1920s to the 1960s public and private schools are even in terms of the average GPA they conferred. But in the 1960s, private schools have experienced GPA inflation at a much faster rate than public schools.  The social scientist in me wondered: Is this disparity due to private schools attracting more talented students, due in part to the money needed to attend a private institution, or “something else.”  The study–which I read–gives us an answer via a simple regression model:

Our database indicates that current grades at an institution can be roughly predicted by either of the following two formulae:

Average GPA = 2.8 +0.005*SEL + (if school is private add 0.2)

Average GPA = 2.8 + 0.001*(SATMV-850) + (if school is private add 0.1)

where SATMV is the combined average Math and Verbal SAT score of students and SEL is a selectivity measure that represents the average of the percentage of students with high school GPAs above 3.75, the percentage of students who graduated in the upper 10% of their high school class, and the percentage of student applicants rejected.

Here is the key:

The above two equations suggest that private schools are grading 0.1 to 0.2 higher on a 4.0 scale for a given talent level of student. Since the evidence indicates that private schools in general educate students no better than public schools (Perscarella and Ternzini, 1991), private schools are apparently conferring small but measurable advantages to their students by more generous grading. Private schools also have on average students from wealthier families, and the effect of our nation’s ad hoc grading policy is to confer unfair advantages to those with the most money.

That’s a fairly stunning indictment of the public-private education divide, if true.  One item that’s not addressed in the paper is the theoretical mechanism driving this result: Why, beginning in the 1960s, did private schools start conferring higher GPAs relative to public schools?  I’m sure a cynical argument could be made that the more a student pays for their education the more pressure that is felt by faculty to award higher grades (the “product,” in this theory).  Though not addressed in this manner in the research report, there is some additional evidence that suggests this might be true.  Specifically, they report that public flagships have experienced grade inflation at a higher rate than their public brethren (resembling private schools in this regard).

Now the author is careful to point out that there is significant variability in these estimates and that a number of schools don’t follow these trends (public or private).  In particular, they note that private liberal arts colleges tend to award grades more rigorously.  All in all I recommend reading the full research report–it’s fairly short and contains a number of interesting bits of information relative to grade inflation.