  Estimates of inflationary parameters from the CMB B-mode polarization
spectrum on the largest scales depend on knowledge of the reionization history,
especially at low tensor-to-scalar ratio. Assuming an incorrect reionization
history in the analysis of such polarization data can strongly bias the
inflationary parameters. One consequence is that the single-field slow-roll
consistency relation between the tensor-to-scalar ratio and tensor tilt might
be excluded with high significance even if this relation holds in reality. We
explain the origin of the bias and present case studies with various tensor
amplitudes and noise characteristics. A more model-independent approach can
account for uncertainties about reionization, and we show that parametrizing
the reionization history by a set of its principal components with respect to
E-mode polarization removes the bias in inflationary parameter measurement with
little degradation in precision.
