One of the challenges in the research is describing the school readiness gap in a developmental metric that allows clear communication with policy makers, programs, and families. In much existing literature, achievement differences are defined in terms of an effect size that is calculated as the proportion of a standard deviation on a standardized, normed test score. These effect sizes are used to describe the magnitude of the gap and the impacts of interventions aimed at narrowing the gap. Although this way of describing the gap provides researchers with a common metric to describe effects, it can be abstract for policymakers. Therefore, researchers (e.g., Layzer & Price, 2008) have investigated ways to describe the gap between the performance of low income samples and the norming samples in terms of the difference in months of development. Using this metric, in their examination of several large samples of low-income children, Layzer and Price found that at age 4, the gap in cognitive and language development between low-income children and norms for their age group is approximately one year of development. In other words, at age 4, low-income children are, on average, one year behind in development, based on the average norm score for their same-age peers. Describing the gap as reflecting one year of development rather than as one standard deviation offers an interpretation of the difference that is more easily understood. It is by no means the only way to accomplish this goal. It will be important to continue this work to translate differences from abstract effect sizes into meaningful developmental metrics.
Determining a metric for describing the achievement gap between children from low-income and better-resourced homes is an important methodological challenge. Substantively, the challenge for the early childhood field is to identify interventions that reduce the gap, however it is measured. That is, we want to design interventions that are effective at decoupling family income and child achievement. If we are successful, there will continue to be individual variation in children's achievement, but it will not be a function of family income.
Many studies have shown that school readiness interventions make progress toward narrowing the gap, but do not close it all the way. A wide range of hypotheses have been posited to explain the size and persistence of the gap. One hypothesis is that early childhood interventions are not intensive enough; many last only nine months and involve no more than half a day of exposure period, which may not be a sufficient amount of time to make up a one-year gap. Further, many interventions last only one year and occur in the year before children enter school. Some researchers suggest that the gap is so wide by the time children are age 4 that intervention needs to begin earlier as well as last longer. Questions also have been raised about the quality of the instructional practices in early childhood settings and whether most children are receiving the kinds of experiences that are most effective for improving their school readiness outcomes. Similarly, questions arise about approaches to improving the level at which providers implement interventions. In addition, researchers have questioned whether interventions provided in early care and education settings can be expected to close the gap entirely, when other environmental factors related to the gap are still at play.
Despite expectations that early childhood interventions will provide a boost that enables children to be successful in school, there is mixed evidence that impacts are sustained. Again, a number of hypotheses have been proposed to explain why impacts are not sustained. Some researchers suggest that, because students still remain below grade level despite making gains, the boost obtained from preschool may not be sufficient to enable students to keep pace at the next grade level. As a result, some researchers argue that students continue to require enhanced instructional supports in elementary school in order for impacts to be sustained. Another hypothesis is that gains are not sustained because many low-income children enter under-resourced kindergarten and elementary school classrooms that provide little support for earlier gains. Data suggest that regardless of children's achievement level at the start of kindergarten, their growth rates during elementary school are similar, suggesting that children who start behind, stay behind (Layzer & Price, 2008). In fact, some research suggests the achievement gap widens over time. As students are expected to learn new skills, which build on foundational skills that may have not been established, students may fall further behind their on-grade-level peers.
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