Methodology
Data: SK-I (Apr 1996 - Jul 2001, 1497.4 d), SK-II (Oct 2002 - Oct 2005, 793.7 d), SK-III (Jul 2006 - Aug 2008, 562.0 d); 22,500 t fiducial volume. Detection channels: neutrino-electron elastic scattering (all flavors) and inverse beta decay; MSW oscillation effects in Sun and Earth included. Energy window 19 (20 for SK-II) to 55 MeV chosen to exclude solar neutrinos and spallation below, and the recoil-spectrum endpoint above. Cuts: spallation likelihood, Cherenkov opening angle (38-50 deg), gamma-ray distance-to-wall, pion-like pattern, multi-ring, decay-electron pairs — leaving 317 candidates (from 2108 after the basic cuts). Signal extraction: chi-square fit of the cos(theta_Sun) angular distribution against expected elastic-scattering and inverse-beta-decay shapes plus an iteratively-determined isotropic background; systematic errors 5.6-6.4% per phase. Remaining spallation background estimated 4.7 +/- 4.3 (SK-I/III) and 0.2 +/- 1.7 (SK-II) events; ~1 solar-neutrino event.