At the Bureau International des Poids et Mesures (BIPM), Sèvres, France, the capacitance unit is linked to a dc quantized Hall resistance (QHR) by means of impedance bridges and stable transfer standards, establishing traceability to the defining fundamental constants of the International System of Units (SI). In the ohm-farad linking chain, a key step is the determination of the frequency dependence of two ac resistors. In this article, it is described how this determination was recently made using two different impedance bridges and how the results are validated using regression analysis.