"In response, private schools – more so than public schools – raised their grades. In the words of one late faculty member from Dartmouth, “we began systematically to inflate grades, so that our graduates would have more A's to wave around”(Perrin, 1998)
. The GPA gap between the private and public schools widened through the 1970s, and has stabilized since the 1980s.
Looking at finer scale variability, we find that colleges and universities have, without any collective consultation or external pressure, collectively created an ad hoc national grading scale based on school selectivity. 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. 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
Even when you account for the students at private schools being "smarter" (higher SAT scores, higher high school GPAs) grades are highly inflated. A 3.0 from a public school should give you a small boost over someone with a 3.0 from an Ivy.