Recent advances in the statistical theory of hierarchical linear models should enable important breakthroughs in the measurement of psychological change and the study of correlates of change. A two-stage model of change is proposed here. At the first, or within-subject stage, an individual's status on some trait is modeled as a function of an individual growth trajectory plus random error. At the second, or between-subjects stage, the parameters of the individual growth trajectories vary as a function of differences between subjects in background characteristics, instructional experiences, and possibly experimental treatments. This two-stage conceptualization, illustrated with data on Head Start children, allows investigators to model individual change, predict future development, assess the quality of measurement instruments for distinguishing among growth trajectories, and to study systematic variation in growth trajectories as a function of background characteristics and experimental treatments.

This study examines the role of exposure to speech in children's early vocabulary growth. It is generally assumed that individual differences in vocabulary depend, in large part, on variations in learning capacity. However, variations in exposure have not been systematically explored. In this study we characterize vocabulary growth rates for each of 22 children by using data obtained at several time points from 14 to 26 months. We find a substantial relation between individual differences in vocabulary acquisition and variations in the amount that particular mothers speak to their children. The relation between amount of parent speech and vocabulary growth, we argue, reflects parent effects on the child, rather than child-ability effects on the parent or hereditary factors. We also find that gender is an important factor in rate of vocabulary growth.The work reported here was supported by grants from the Spencer Foundation and the Smart Family Foundation to Janellen Huttenlocher. We thank Peter R. Huttenlocher and James W Stigler for their helpful comments on the manuscript.

the 1990s as two of the worst schools in Chicago in terms of math and reading achievement. Only two miles apart, the schools are in bordering neighborhoods and appear similar in many ways. Both enrolled nearly 100% minority students from families considered low income.During the 1990s, both launched an array of initiatives aimed at boosting student achievement. Hancock moved impressively forward, while Alexander barely moved the needle on improvement. How did Hancock "beat the odds" while Alexander failed to do so?This puzzle led us to undertake a systematic longitudinal investigation of hundreds of elementary schools in Chicago, just like Alexander and Hancock. Beginning in 1990, the Consortium on Chicago School Research initiated an intensive longitudinal study of the internal workings and external community conditions that distinguished improving elementary schools from those that failed to improve. That unique 15-year Organizing Schools for ImprovementResearch on Chicago school improvement indicates that improving elementary schools requires coherent, orchestrated action across five essential supports.

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