Why not to calculate Cp for unilateral tolerance?

Answer 1

Calculating Cp for unilateral tolerance is not advisable because Cp assumes a symmetrical distribution of measurements around the target value, which is not the case with unilateral tolerances. Unilateral tolerances only allow variation in one direction (either upper or lower), leading to a lack of symmetry and potential underestimation of process capability. This can result in misleading conclusions about the process's ability to meet specifications, as it does not account for the increased risk of nonconformance. Therefore, alternative metrics, like Cpk, which consider the mean and variability in relation to the specific tolerance limits, are more appropriate for unilateral tolerances.