Does day care help low-income children stay in school and...

Does day care help low-income children stay in school and hold good jobs later in life? The Carolina Abecedarian Project (the name suggests the $\mathrm{ABC}$ s) has followed a group of 111 children since 1972. Back then, these individuals were all healthy but low-income black infants in Chapel Hill, North Carolina. All the infants received nutritional supplements and help from social workers. Half were also assigned at random to an intensive preschool program. ${ }^{36}$ Explain how each of the four principles of experimental design was used in this study.

Key Concepts

Blocking

Blocking involves grouping subjects with homogeneous characteristics to reduce variability and isolate the effects of the treatment. In this study, all participants were selected based on shared characteristics, such as being low-income black infants, which helps control for extraneous variables. This form of blocking ensures that differences in long-term outcomes are more likely due to the experimental intervention rather than other demographic or socioeconomic differences.

Replication

Replication means including a sufficient number of subjects in each group to reliably detect the effect of the treatment and to ensure that the findings are robust. In the Carolina Abecedarian Project, 111 children were followed over a long period, providing replication through multiple individuals undergoing the same treatment conditions. This replication across many subjects increases the statistical power and validity of the study's conclusions.

Randomization

Randomization involves assigning participants to different groups in a random manner to reduce bias and ensure that differences between groups are due to the treatment rather than preexisting differences. In this study, half of the children were randomly assigned to receive the intensive preschool program, which helps to balance out any unknown factors that might influence later outcomes between the treatment and control groups.

Control

The control principle requires having a comparison group that does not receive the experimental treatment, ensuring that any differences observed can be attributed to the intervention. In this study, while all children received nutritional supplements and social services, only half received the additional day care program. The children who did not receive the intensive preschool program serve as a control group, allowing researchers to isolate the effects of the day care intervention.

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