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chap21_2010

# chap21_2010 - Chapter 21 Measurement Analysis Measurement...

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Chapter 21 Measurement Analysis

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Measurement It is important to define and validate the measurement system before collecting data. Without measurement we only have opinions The measurement system is the complete process used to obtain measurements. Measurement error is inevitable. We must identify, evaluate, and control the sources of measurement error. Any variation can be attributed to either the characteristic that is being measured, or the way the measurements are being taken.
Measurement ε σ σ σ + + = 2 2 2 t measuremen item total 2 2 2 2 others process individual t measuremen σ σ σ σ + + =

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Sources of error Measurement error = the effect of all sources of measurement variability that cause an observed value to deviate from the true value being measured. Measuring instrument errors: Accuracy Linearity Stability Precision Measuring system errors Repeatability Reproducibility
Defining Error Accuracy = difference between the observed average and reference value. Linearity = change in accuracy across the expected operating range of the measuring instrument Stability = consistency in the measurement over time Precision = standard deviation between measurements Repeatability = variation obtained by one operator measuring the same characteristic with the same instrument Reproducibility = variation in the average of measurements taken by different operators using the same instrument.

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How to measure error? Multiple measurements of one single characteristic Precision: Standard deviation among measurements Accuracy: Difference between the observed average and the reference value
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chap21_2010 - Chapter 21 Measurement Analysis Measurement...

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