 Systematic-error cancellation by reversal

If a wanted response changes sign on reversing a controlled setting while an offset remains unchanged, the difference of the reversed readings isolates the response. For $V_{\pm }=\pm RI+V_0$, the estimator $(V_{+}-V_{-})/(2I)$ cancels $V_0$. The method requires offset stability between readings. It does not cancel a common current-scale calibration error, and it does not remove even-in-current Joule heating. A useful reversal test specifies which physical effects change sign, then tests the remaining effects independently.