Kevin Carey:

You see it all the time, in the brochures and advertisements from liberal arts colleges and other non-gargantuan institutions. “Small class sizes,” they promise, and for good reason, because everyone knows that small classes are better than large. No cavernous lecture halls where the professor is little more than a distant stick figure, they say — raise your hand here, and someone will stop and listen. Plus, he or she will be a real professor, the genuine tenure-track article, not a part-timer or grad student but someone who really knows his or her stuff. Because everyone knows that real professors are better than the other kind.
Except, they don’t.
Nobody actually knows whether small classes are better than large. Pascarella and Terenzini’s How College Affects Students, the bible of such matters, says “We uncovered 10 studies that focus on the effects of class size on course learning. All of the investigations are quasi-experimental or correlational in design …. Unfortunately, five of the studies used course grade as the measure of learning … the conflicting evidence and continuing methodological problems surrounding this small body of research make it difficult to form a firm conclusion.”