The correct answer is B. xₘ − x . The CSSBB source defines bias as the difference between the output of the measurement system and the true value . It is often referred to as accuracy . The formal definition in the instructor text is the average measured value minus the value of the standard or true value.

Using the notation in this question, where xₘ is the measured value and x is the true value, the bias is simply:

Bias = xₘ − x

This gives both the size and the direction of the bias. If the result is positive, the gage overestimates. If it is negative, the gage underestimates. That is important in Six Sigma measurement system analysis because the team needs to know not only how far off the measurement system is, but also whether it consistently reads high or low. The ratio in option A is not the definition of bias, option C is a relative or percent-type expression, and option D removes the direction of the error. Therefore, the verified CSSBB answer is B .