A measurement of Rb using a lifetime-mass tag

ALEPH's published measurement of Rb = Γ(Z -> bb)/Γ(Z -> hadrons) using a lifetime tag is updated using the full LEP 1 data sample. Considerable effort has been devoted to understanding systematic effects. Charm background is better controlled by combining the lifetime tag with a tag based on the b/c hadron mass difference. Furthermore, the algorithm used to reconstruct the event primary vertex is designed so as to reduce correlations between the two hemispheres of an event. The value of Rb is measured to be 0.2167 +/- 0.0011 (stat) +/- 0.0013 (syst).