This book provides a systematic exposition of a group-based statistical method for analyzing longitudinal data in the social and behavioral sciences and in medicine. The methods can be applied to a wide range of data, such as that describing the progression of delinquency and criminality over the life course, changes in income over time, the course of a disease or physiological condition, or the evolution of the socioeconomic status of communities. Using real-world research data from longitudinal studies, the book explains and applies this method for identifying distinctive time-based progressions called developmental trajectories. Rather than assuming the existence of developmental trajectories of a specific form before statistical data analysis begins, the method allows the trajectories to emerge from the data itself. Thus, in an analysis of data on Montreal school children, it teases apart four distinct trajectories of physical aggression over the ages 6 to 15, examines predictors of these trajectories, and identifies events that may alter the trajectories.

Aimed at consumers of statistical methodology, including social scientists, criminologists, psychologists, and medical researchers, the book presents the statistical theory underlying the method with a mixture of intuition and technical development.

Acknowledgments                                                     ix
1 Introduction and Rationale                                            1
Part I. Laying Out the Basic Model
2 The Basic Model                                                      23
3 Groups as an Approximation                                           45
4 Model Selection                                                      61
5 Posterior Group-Membership Probabilities                             78
Part II. Generalizing the Basic Model
6 Statistically Linking Group Membership to Covariates                 95
7 Adding Covariates to the Trajectories Themselves                    118
8 Dual Trajectory Analysis                                            141
9 Concluding Observations                                             171
References                                                         187
Index                                                               199