Nevertheless, one can draw from 2003 and 2006 samples to find good matches and construct reference groups for students tested in 2000. We match using propensity' score P2000 = P(T=200Q\X), reflecting the propensity to be in the PISA 2000 sample. Two propensity scores must be estimated: one measuring a propensity of being in a vocational school in 2000 for students tested in 2003 or 2006, and a second for being in a general (or mixed vocational-general) school in 2000 for students tested in 2003 or 2006.