Surveys often sample adults across a broad range of ages, measuring the same outcomes in several interviews spaced during a period of years and comparing the changes observed across segments of the adult life course. Put in sequence, those change vectors provide a composite image of the outcome's life course trajectory. To illustrate, the authors estimate depression vectors in a sample of U.S. adults ages 18 and older at baseline in 1995, with follow-up interviews in 1998 and 2001. They show the vector equations and their graphs and also their synthetic-cohort projection. The authors introduce the trend-function and virtual-cohort projection, showing how they provide tests of ``convergence'' and other hypotheses about trajectories and trends. Results show depression dropping and then rising across adulthood more steeply than suggested by cross-sectional differences among age groups. They also indicate a rise and fall in age-specific levels of depression across cohorts.
