A couple of fine new studies attest to the importance of quality instruction for preschoolers—and the dizzying (“stunning” says one research team) range of bad-to-excellent offerings in today’s early childhood programs and centers. “There is no evidence whatsoever,” we read, “that the average preschool program produces benefits in line with what the best programs produce.” The problem is that the input-based measures long used as proxies for quality by the early-childhood community—teacher credentials, child-teacher ratios, etc.—do not explain much of the variance….
