Longitudinal studies conducted by the same researcher are coded for the span of all years in which the data collections took place. Waves of longitudinal studies conducted by different researchers are only coded for the time of follow-up data collection. Race and ethnicity are coded as defined by the contributor. Not asked No data on race was collected. Not compiled Information on race is available from the data but has not been compiled by the researcher.
Every age group sampled during the life of the study is coded.
The number of generations in the study sampled are coded. For example, if children or parents of target subjects are included in the study, then the study covers two generations. If both are in the study, along with the target subjects, then the study covers three generations. If a study is of a specific socioeconomic group or groups, such as the working class or middle class, these categories are coded. Subjects who are retired or unemployed are listed according to the status of their previous occupational category.