  We experimentally investigate an optical clock based on $^{171}$Yb ($I=1/2$)
atoms confined in an optical lattice. We have evaluated all known frequency
shifts to the clock transition, including a density-dependent collision shift,
with a fractional uncertainty of $3.4 \times 10^{-16}$, limited principally by
uncertainty in the blackbody radiation Stark shift. We measured the absolute
clock transition frequency relative to the NIST-F1 Cs fountain clock and find
the frequency to be 518 295 836 590 865.2(0.7) Hz.
