The analysis of residuals in the calibration or inverse-regression problem is considered. A statistic is proposed that is appropriate to the specific use of the regression equation in this context—namely, estimatingxfromy. The statistic, similar in spirit to Cook's distance, may be used to measure the influence a particular observation may have on future estimates ofxfrom the calibration curve. It may also be used to measure the differences between estimates ofxobtained using calibration curves based on different models. An example is given.