A vital component of continuous improvement in production environments is measuring performance. For instance, in the total productive maintenance philosophy, overall equipment effectiveness is used as a key performance indicator. However, the validity of this and other performance indicators is a question when the measurements cannot be obtained accurately. In this study, measurements on production line speeds and unplanned stoppage durations are assumed to have measurement errors. A balance equation is developed for production loss amounts by formulating the relationship between accurate measurements and measurements with error. To obtain corrected values of the line speed and unplanned stoppage durations by using the measurements given by an operator, a nonlinear mathematical model is developed. It is shown that the solution to the mathematical formulation is unique and global optimum. The problem and its proposed solution is presented by a real world example from a PVC profile manufacturing, which motivated this research. A particular advantage of the proposed approach would be in obtaining better measures for process performance.