The accuracy of the SeaWinds scatterometer's vector winds is assessed through comparison with research vessel observations. Factors that contribute to uncertainty in scatterometer winds are isolated and examined as functions of wind speed. For SeaWinds on QuikSCAT, ambiguity selection is found to be near perfect for surface wind speed (w) > 8 m s-1; however, ambiguity selection errors cause directional uncertainty to exceed 20¿ for w 7.5 m s-1 (w > 18 m s-1). This approach also shows that spatial variability in the wind direction, related to inexact spatial co-location, is likely to dominate rms differences between scatterometer wind vectors and in situ comparison measurements for w > 4.5 m s-1. The techniques used herein are applicable to any validation effort with uncertainty in the comparison data set or with inexact co-location.