Numerical error analysis is applied to typical 40Ar/39Ar data. It extends previous error treatments to al! possible variables used in the calculation of ages of samples and shows that in some cases minor corrections may contribute significantly to age uncertainties. Isochron plots using 36Ar as a reference isotope are not suitable for data analysis - the correlation plot with 40Ar as a reference is preferred. The irradiation parameter, J, is the primary limitation on the precision of age determinations. Close control of this parameter permits age precision of O. I % (! a) but O. 5 % is typical.