  We report an uncertainty evaluation of an optical lattice clock based on the
$^1S_0\leftrightarrow^3P_0$ transition in the bosonic isotope $^{174}$Yb by use
of magnetically induced spectroscopy. The absolute frequency of the
$^1S_0\leftrightarrow^3P_0$ transition has been determined through comparisons
with optical and microwave standards at NIST. The weighted mean of the
evaluations is $\nu$($^{174}$Yb)=518 294 025 309 217.8(0.9) Hz. The uncertainty
due to systematic effects has been reduced to less than 0.8 Hz, which
represents $1.5\times10^{-15}$ in fractional frequency.
