Little Sleepers: Long-Term Effects of Preschool

09/28/2017 10:02 am ET

In education research, a “sleeper effect” is not a way to get all of your preschoolers to take naps. Instead, it is an outcome of a program that appears not immediately after the end of the program, but some time afterwards, usually a year or more. For example, the mother of all sleeper effects was the Perry Preschool study, which found positive outcomes at the end of preschool but no differences throughout elementary school. Then positive follow-up outcomes began to show up on a variety of important measures in high school and beyond.

Sleeper effects are very rare in education research. To see why, consider a study of a math program for third graders that found no differences between program and control students at the end of third grade, but then a large and significant difference popped up in fourth grade or later. Long-term effects of effective programs are often seen, but how can there be long-term effects if there are no short-term effects on the way? Sleeper effects are so rare that many early childhood researchers have serious doubts about the validity of the long-term Perry Preschool findings.

I was thinking about sleeper effects recently because we have recently added preschool studies to our Evidence for ESSA website. In reviewing the key studies, I was once again reading an extraordinary 2009 study by Mark Lipsey and Dale Farran.

The study randomly assigned Head Start classes in rural Tennessee to one of three conditions. Some were assigned to use a program called Bright Beginnings, which had a strong pre-literacy focus. Some were assigned to use Creative Curriculum, a popular constructive/developmental curriculum with little emphasis on literacy. The remainder were assigned to a control group, in which teachers used whatever methods they ordinarily used.

Note that this design is different from the usual preschool studies frequently reported in the newspaper, which compare preschool to no preschool. In this study, all students were in preschool. What differed is only how they were taught.

The results immediately after the preschool program were not astonishing. Bright Beginnings students scored best on literacy and language measures (average effect size = +0.21 for literacy, +0.11 for language), though the differences were not significant at the school level. There were no differences at all between Creative Curriculum and control schools.

Where the outcomes became interesting was in the later years. Ordinarily in education research, outcomes measured after the treatments have finished diminish over time. In the Bright Beginnings/Creative Curriculum study the outcomes were measured again when students were in third grade, four years after they left school. Most students could be located because the test was the Tennessee standardized test, so scores could be found as long as students were still in Tennessee schools.

On third grade reading, former Bright Beginnings students now scored significantly better than former controls, and the difference was statistically significant and substantial (effect size = +0.27).

In a review of early childhood programs at, our team found that across 16 programs emphasizing literacy as well as language, effect sizes did not diminish in literacy at the end of kindergarten, and they actually doubled on language measures (from +0.08 in preschool to +0.15 in kindergarten).

If sleeper effects (or at least maintenance on follow-up) are so rare in education research, why did they appear in these studies of preschool? There are several possibilities.

The most likely explanation is that it is difficult to measure outcomes among four year-olds. They can be squirrely and inconsistent. If a pre-kindergarten program had a true and substantial impact on children’s literacy or language, measures at the end of preschool may not detect it as well as measures a year later, because kindergartners and kindergarten skills are easier to measure.

Whatever the reason, the evidence suggests that effects of particular preschool approaches may show up later than the end of preschool. This observation, and specifically the Bright Beginnings evaluation, may indicate that in the long run it matters a great deal how students are taught in preschool. Until we find replicable models of preschool, or pre-k to 3 interventions, that have long-term effects on reading and other outcomes, we cannot sleep. Our little sleepers are counting on us to ensure them a positive future.

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