... vector y − X 1 ´β 1 and the projectionmatrices MPWX 1 and MPWX, respectively. ThusSSR0− SSR 1 = (y − X 1 ´β 1 )(MPWX 1 − MPWX)(y − X 1 ´β 1 )= (y − X 1 ´β 1 )(PPWX− ... Buse (19 92), Bekker (19 94),Bound, Jaeger, and Baker (19 95), Dufour (19 97), Staiger and Stock (19 97),Wang and Zivot (19 98), Zivot, Startz, and Nelson (19 98), Angrist, Imbens, and Krueger (19 99), ... X 1 ´β 1 )(PPWX− PPWX 1 )(y − X 1 ´β 1 ), (8.56)where PPWX 1 and PPWXproject orthogonally on to S(PWX 1 ) and S(PWX),respectively, and MPWX 1 and MPWXare the complementary...