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In the second and later follow-up surveys, 18 of the "extra" schools were used to include all cases with complete base-year data. Samples of former 1972 senior students were also selected from 16 sample augmentation schools; these schools were selected...

School File Documentation - Page 3
by Louise B. Henderson - 1981
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National Longitudinal Study: Base Year, First, Second, and Third ..., Volume 1

Research Triangle Institute. Center for Educational Research and Evaluation, Jay Levinsohn - High school graduates - 1978 - 486 pages

...strata. In the second and later follow-up surveys, 18 of the "extra" schools were used to include all cases with complete base-year data. Samples of former...not included in the original school sampling frame. Samples of students selected from the 16 augmentation schools were not included in the base-year survey...

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