= 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.
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