The waters of Lake Wobegon have flooded U.S. college campuses. A’s — once reserved for recognizing excellence and distinction — are today the most commonly awarded grades in America.

That’s true at both Ivy League institutions and community colleges, at huge flagship publics and tiny liberal arts schools, and in English, ethnic studies and engineering departments alike. Across the country, wherever and whatever they study, mediocre students are increasingly likely to receive supposedly superlative grades.

Such is the takeaway of a massive new report on grade inflation from Stuart Rojstaczer, a former Duke University professor, using data he and Furman University professor Chris Healy collected. Analyzing 70 years of transcript records from more than 400 schools, the researchers found that the share of A grades has tripled, from just 15 percent of grades in 1940 to 45 percent in 2013. At private schools, A’s account for nearly a majority of grades awarded.