  Based on the five-orbital model, we derive the reduced impurity scattering
rate $g=z\gamma/2\pi T_{c0}$ in Sm(Fe$_{1-x}$Ru$_{x}$)AsO$_{0.85}$F$_{0.15}$
from the residual resistivity. At $x=0$, the transition temperature is
$T_{c0}=50$ K. For $0.05<x<0.36$ ($0.84>T_{c}/T_{c0}>0.3$) the obtained value
of $g$ ranges from 1.5 to 2.9, which suggests that the $s_\pm$-wave state
cannot survive. We point out that the magnetoresistance frequently gives an
underestimated value of $g$ in correlated electron systems.
