Measurement error Roland Caulcutt Caulcutt Associates

Two processes 2 Production process Materials Information Products/services Measurement process Data A process is a combination of equipment, people and procedures that work together to produce products and services. All of your data comes from measurement processes.

Measurement error 3 Many people wish to believe that measurement error does not exist. These people measure something once and assume that the measurement is “correct”. They believe that taking a second measurement would be pointless.

Repeat measurements Length (cm) Spread The spread of repeat measurements quantifies the precision of the measurement process.

To assess precision Length (cm) Range We could use:Spread = range of data We use:Spread = 6sd. 6 sd

Precision 6 If the measurements were made by the same person, using the same equipment, then: The spread is a measure of repeatability. If the measurements were made by different people then: The spread is a measure of reproducibility or R&R.

VIM 7 International vocabulary of metrology (VIM) The VIM defines repeatability and reproducibility. If everyone adopted these definitions there would be no ambiguity. Component manufacturers do accept them but many chemical manufacturers don’t. They prefer to focus on the difference that might exist if producer and customer measure the same delivery.

8 How good is your measurement process? The capability of your measurement process will depend on how you intend to use the data it produces. You might use data to help you to: 1.Control a production process 2.Assess the capability of a production process 3.Improve a production process 4.Classify products or services 5.Settle customer/supplier disputes

Relative precision 9 To make your assessment of precision more meaningful you could relate it to the purpose of your measurement. If you are measuring in order to classify product you could express your spread as a % of the specification. If you are measuring in order to improve the production process you could:  Express your spread as a % of your product variation.  Calculate number of distinct categories

10 Because of measurement error you may be left in doubt about the acceptability of the product or service. Not acceptable Not acceptable Acceptable Uncertainty ????? Uncertainty ????? LSL USL Quality Classification of product

11 LSL USL R&R = 100% R&R = 30% R&R = 10% R&R = 0% R&R as a % of specification

GUM 12 Guide to the expression of uncertainty in measurement (GUM) (True value) = (Measured value) + k(SD) It is extremely unlikely that the measured value will be equal to the true value. We make one measurement. We report an interval in which we are confident that the true value will lie.

Bias Length (cm) Bias Bias may be a bigger source of error than poor precision. True value Mean

Thank you for your attention. 14