Measurement error is defined as the effect of all sources of measurement variability that caused an observed or measured value to deviate from the __________.

Measurement error is the discrepancy between a measured value and the true value of the quantity being measured. It represents the combined effect of all sources of variability in a measurement system, quantifying the system's inaccuracy. The goal of Measurement System Analysis (MSA) is to identify and minimize these sources of error to ensure that observed values are as close as possible to the actual, or true, value. This concept is fundamental to assessing the reliability and validity of data collected for process improvement.

A. Standard Deviation: This is a measure of the dispersion or spread of a set of measurements around their own average, not their deviation from a true value.
B. Mean: The mean is the average of the observed measurements. The difference between an observed value and the mean is a deviation, but not the measurement error itself.
C. Median: The median is the middle value in a dataset. It is a measure of central tendency, not the benchmark for accuracy or the true value.

1. Montgomery, D. C. (2013). Introduction to Statistical Quality Control (7th ed.). John Wiley & Sons. In Chapter 8, Section 8-1, "The Measurement Process," it is stated: "The difference between the true value of the characteristic that is being measured and the observed value is the measurement error." (p. 339).

2. The ASQ CSSGB Certification Preparation course materials, based on the Body of Knowledge, define measurement error as the difference between the observed average of measurements and a master or true value. This aligns with the concept of accuracy. (ASQ Six Sigma Green Belt Body of Knowledge, V. Measure Phase).

3. JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement (GUM). In Section B.2.19, "error (of measurement)" is defined as the "measured quantity value minus a reference quantity value." The note clarifies that the reference value is often a "true quantity value." (p. 16).