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Analysis Methods

Impact estimates discussed in this report represent the effect of Head Start on children and parents after one year of program participation.6 Estimates are primarily based on the use of statistical models that control for any random differences in background characteristics between the Head Start and non-Head Start groups. Impacts are presented both for the overall average effects (for the full sample) and for selected subgroups of children and parents. All estimates use weighted data to generalize the findings to the full population of newly entering Head Start children.

Before describing the results, three points are worth emphasizing.

  1. The initial analyses represent only a portion of what is planned for future reports: In looking at child experiences, the current report provides only a partial set of preliminary indicators. Future reports will expand upon the description of the characteristics of the child care settings used by families and explore how child impacts vary with the quality of their early care experience. Additionally, future reports will address an expanded array of outcomes, the impacts of full-day/part-day programs, and other factors that have been shown to influence children’s school readiness, such as teacher characteristics.

  2. The non-Head Start (control) group is not a “no service” group: Parents of children in the control group were not precluded from enrolling their children in other types of preschool or child care arrangements. Consequently, the impact of Head Start is being evaluated against a mixture of alternatives available in the community, ranging from parent care to non-Head Start center-based programs. In some cases, these alternatives may look very much like Head Start, while others may look very different from Head Start. Evaluating Head Start against the current mixture of alternative arrangements isolates the contribution the Federal program is making relative to the array of other child care services currently available to low-income families.

  3. The magnitude of estimated impacts must be viewed in context: This report uses a strict standard for reporting statistical significance. Only those impacts that could be detected with 95 percent confidence are reported as statistically significant. For those outcomes where statistically significant impacts were detected, results are provided in both their “natural” units (e.g., as points on a test score) and as “effect sizes” which provide a common yardstick for comparing across the different outcomes as well as to other research studies. When no significant impact was detected, effect sizes are not reported. For this report we have adopted the following conventions for interpreting effect sizes. Effect sizes of less than 0.2 are considered small, between 0.2 and 0.5 are considered a moderate impact, and over 0.5 are considered large impacts. For the most part, effect sizes from the current analysis are in the range of small to moderate. In considering the effect sizes, readers should keep in mind that:

    1. These findings represent the impact of Head Start after a single year of participation.

    2. There were some deviations from perfect random assignment that may affect the size and statistical significance of estimated impacts.

    3. Any judgment about the importance of the reported impact estimates must consider both the level of gains that children can be expected to achieve within a relatively short period of time and the size of effects that have been found in other early childhood and educational research studies.



6 These are the average impact of access to Head Start, often referred to as “intent to treat” impact estimates. Additional analysis on the children and parents who actually participated in the program (referred to as the “impact on the treated”) are presented in Appendices 5-8. (back)

 

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