Consider the issue of Intensity and Duration.

Intensity- the amount of the program a child receives on a daily basis.

Duration- how long the child attends the program.

Unfortunately, little is known about the degree of intensity needed to achieve beneficial outcomes for preschoolers.

However, research has shown -

“The effectiveness of any early education program depends on how much of the integral quality ingredients children receive.” (see QPPS and NAEYC comparison).

“The more intensity and the longer the duration of the program, the more contact with these ingredients children have.”

“One relevant study of intensity examined the effects of full-day versus half-day kindergarten. Children who attended the full-day kindergarten scored significantly higher than half-day participants on school readiness indicators, as rated by their first grade teachers.”

“These gains were attributed somewhat to teaching methods and style. In this study, children in half-day kindergarten spent more time in child-initiated activities and stimulating interactions with their teachers. Children in half-day kindergarten spent a higher proportion of their time in more passive teacher-directed activities aimed at the entire group.”
 

“Although preschool programs that have greater intensity and duration appear to be more effective at increasing school readiness, some shorter and intense model programs implemented have also been found to be highly effective. Because it seems highly unlikely that preschool programs in the real world will ever consistently match the quality of model programs implemented under near ideal conditions, increasing their intensity and duration might be viewed as one way of increasing their overall effectiveness.”

Source: A Vision for Universal Preschool Education, Zigler, Gilliam, Jones, 2006