Cognitive scores of infants in a study of early childhood intervention. The 103 infants from low income African American families were divided into a treatment group (58 infants) and a control group (45 infants). Starting at 0.5 years of age the infants in the treatment group were exposed to an enriched environment. Each infant's cognitive score on an age-specific, normalized scale was recorded at ages 1, 1.5, and 2 years.
groupedData object contains the following columns
An ordered factor of the id number for each infant.
A numeric cognitive score.
The age of the infant at the measurement.
A factor with two levels,
indicating if the infant is in the early childhood intervention
Singer, Judith D. and Willett, John B. (2003), Applied Longitudinal Data Analysis, Oxford University Press. (Ch. 3)