Dear Rick,

The method described does not cover the situation of data including negative measurements.

The formulas
aL=median{yi - bU xi}
aU=median{yi - bL xi}
must then be modified to
aL=median[min({yi - bU xi},{yi - bL xi}]
aU=median[max({yi - bU xi},{yi - bL xi}]
in order to ensure that, for any (xi,yi), the upper bound of the value aU is always greater than the lower bound aL.

Best regards
Jean-Christophe Lemarié