The largest federal preschool program is Head Start, which "provides comprehensive educational, social, health, and nutritional services to low-income preschool children and their families." From 2002 through 2008, the U.S. Department of Health & Human Services conducted a nationally representative study of the effects of Head Start. This was an experimental study, which means that the results show cause and effect. The study found "there were initial positive impacts from having access to Head Start, but by the end of 3rd grade there were very few impacts," and among these, some were positive and some were negative with no "clear pattern" in either direction.