INTRODUCTION

A longitudinal, or panel, data set is one that follows a given sample of individuals over time, and thus provides multiple observations on each individual in the sample. Panel data have become widely available in both the developed and developing countries. In the United States, two of the most prominent panel data sets are the National Longitudinal Surveys of Labor Market Experience(NLS) and the University of Michigan's Panel Study of Income Dynamics(PSID).

The NLS was initiated in 1966. The surveys include data about a wide range of attitudes, behaviors, and events related to schooling, employment, marriage, fertility, training, child care, health, and drug and alcohol use. The original four cohorts were men aged 45 to 59 in 1966, young men aged 14 to 24 in 1966, women aged 30 to 44 in 1967, and young women aged 14 to 24 in 1968. Table 1.1 summarizes the size and the span of years each group of these original samples has been interviewed, as well as the currently ongoing surveys (the NLS Handbook 2005 U.S. Department of Labor, Bureau of Labor Statistics).In 1979, the NLS expanded to include a nationally representative sample of 12,686 young men and women who were 14 to 22 years old. These individuals were interviewed annually through 1994 and are currently interviewed on a biennial basis (NLS79). In 1986, the NLS started surveys of the children born to women who participated in the National Longitudinal Survey of Youth 1979 (NLS79 Children and Young Adult).