  In this paper we do a detailed $\chi^2$-analysis of the 848 days of
Super-Kamiokande(SK) atmospheric neutrino data under the assumptions of
$\nu_\mu - \nu_\tau$ oscillation and neutrino decay. For the latter we take the
most general case of neutrinos with non-zero mixing and consider the
possibilities of the unstable component in $\nu_\mu$ decaying to a state with
which it mixes (scenario (a)) and to a sterile state with which it does not mix
(scenario (b)). In the first case $\Delta m^2$ (mass squared difference between
the two mass states which mix) has to be $>$ 0.1 $eV^2$ from constraints on $K$
decays while for the second case $\Delta m^2$ can be unconstrained. For case
(a) $\Delta m^2$ does not enter the $\chi^2$-analysis while in case (b) it
enters the $\chi^2$-analysis as an independent parameter. In scenario (a) there
is \dm averaged oscillation in addition to decay and this gets ruled out at
100.0% C.L. by the latest SK data. Scenario (b) on the other hand gives a
reasonably good fit to the data for \dm $\sim 0.001 ~eV^2$.
