Since both decay series contain a unique set of intermediate radioactive isotopes, and because each has its own half-life, independent age calculations can be made from each (Dalrymple 20).The presence of a stable lead isotope that is not the product of any decay series (204Pb) allows lead isotopes to be normalized, allowing for the use of isochrons and concordia-discordia diagrams as dating tools.
This is because Pb, causing the isochron to decrease in slope with increasing age.
The use of lead isotope ratios makes this isochron self-checking.
A large scattering of measurements would indicate the sample is multi-stage rather than single-stage, making the isochron unreliable.
Two other characteristics of lead isotope measurements make it superior to other methods.
First, measuring the isotope ratio of a single element can be done much more precisely than measuring isotope ratios of two differing elements.