= Common-offset cancellation by signal reversal
{title2=$S=(y_+-y_-)/2,\quad b=(y_++y_-)/2$}
If a controlled reversal changes the desired <signal>'s sign while leaving an additive offset unchanged, readings $y_+=S+b$ and $y_-=-S+b$ separate them by half-difference and half-sum. Offset drift, gain changes, hysteresis or a background that also reverses violate the simple cancellation. Interleaving polarities and independent blank measurements test those assumptions; <signal> reversal does not cancel an arbitrary <systematic error>.
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