  We performed a statistical analysis of the astrometric errors for the major
asteroid surveys. We analyzed the astrometric residuals as a function of
observation epoch, observed brightness and rate of motion, finding that
astrometric errors are larger for faint observations and some stations improved
their astrometric quality over time. Based on this statistical analysis we
develop a new weighting scheme to be used when performing asteroid orbit
determination. The proposed weights result in ephemeris predictions that can be
conservative by a factor as large as 1.5. However, the new scheme is more
robust with respect to outliers and better handles faint detections.
