  Thermoelectric semiconducting materials are often evaluated by their
figure-of-merit, zT. However, by using zT as the metric for showing
improvements, it is not immediately clear whether the improvement is from an
enhancement of the inherent material property or from optimization of the
carrier concentration. Here, we review the quality factor approach which allows
one to separate these two contributions even without Hall measurements. We
introduce practical methods that can be used without numerical integration. We
discuss the underlying effective mass model behind this method and show how it
can be further advanced to study complex band structures using the Seebeck
effective mass. We thereby dispel the common misconception that the usefulness
of effective band models is limited to single parabolic band materials.
