History is littered with examples of bad decisions, made on bad measurement data, and their very expensive consequences. In 1999, NASA lost the Mars Orbiter, which was designed to study the Martian climate & atmosphere. It seems the engineering team used English units of measurement, whilst NASA used the metric system. NASA took a $125 million dollar hit on the lost in space Mars Climate Orbiter.
We don’t need to travel to NASA to observe bad decisions made on the basis of poor measurement data. Decisions about process changes and parts’ conformance to specifications are based on measurements or inspection. Measurement itself is a process and like any process it has the potential for variation or error. Variation in the measurement system comes from many sources including people, materials and the environment in which the measurement is taken. Very often our measurements exhibit unacceptable variation and we don’t even know about it.
The risk to the organisation is twofold: firstly, a false reject, which cost the organisation money, and secondly and much more seriously a false pass, which may result in a customer complaint, and worst case a product recall and significant damage to the business. Time and again I have witnessed operators rejecting good parts, and they don’t even know it. Also, more unfortunately, I have witnessed customer complaints, when parts are returned from customers for 100% inspection (or worse, scrap!) after the customer has discovered non-compliant parts in the order.
How can we assess our measurement system? It’s typically a quick and easy, and very informative exercise to conduct what’s called a ‘Gauge R&R’ exercise. This formal technique assesses how likely the assessor (i.e. the person taking the measurement) is to get the same result, when using the same gauge to measure the same characteristic of the same part, repeatedly. This is known as assessing for repeatability, the first R. Gauge R&R also assesses how likely two or more assessors are to get the same result when using the same gauge to measure the same characteristic of the same part, repeatedly. This is known as assessing for reproducibility, the second R. The exercise provides a data set on the accuracy of your measurement system. Based on the data, you can decide whether or not the measurement system needs to be improved or updated.
If you don’t have access to Minitab or JMP, there are many cheap and cheerful Excel add-on packages that can enable you to conduct Gauge R&R exercises. The investment in regular Gauge R&R exercises on key processes measurements more than pays for itself. If you’d like advice on how to go about it, just give me a call on +353 91 870 708.
Principal Consultant, Lean Ireland.
© Bernie Rushe, BSc, CPIM, Dip SA, MSc, Black Belt