Whose Measurement is Right?

Every company I’ve worked for inspects the product it receives from its suppliers to determine conformance to requirements. The process is variously referred to as incoming inspection or receiving inspection.

Sometimes the receiving inspection process identifies a lot of product that fails to conform to requirements. That lot is subsequently classified as nonconforming material and quarantined for further review. There are many reasons why a lot of product may be classified as nonconforming. Here I wish to focus just on reasons having to do with measurement.

Once a company discovers nonconforming parts, it usually contacts its supplier to share that information. It is not unusual, however, for the supplier to push back when their data for the lot of product shows it to be conforming. So, how can a given lot of product be both conforming and nonconforming? Who is right?

We need to recognize that measurement is a process. The measured value is an outcome of this process. It depends on the measurement tool used, the skill of the person making the measurement and the steps of the measurement operation. A difference in any of these factors will show up as a difference in the measured value.

It is rare that a measurement process is the same between a customer and its supplier. A customer may use different measurement tools than its supplier. For example, where the customer might have used a caliper or micrometer, the supplier may have used an optical comparator or CMM. Even if both the customer and the supplier use the same measurement tool, the workers using that tool are unlikely to have been trained in its use in the same way. Finally, the steps used to make the measurement, such as fixturing, lighting and handling the part, which often depend on the measurement tool used, will likely be different, too.

Thus, more often than not, a measurement process between a supplier and a customer will be different. Each measured value is correct in its context—the supplier’s measurement is correct in its context, as is the customer’s measurement in its context. But because the measurement process is different between the two contexts, the measured values cannot be compared directly with one another. So it is possible that the same lot of product may be conforming per the supplier’s measurements and nonconforming per the customer’s measurements.

But why are we measuring product twice: once by the supplier and again by the customer? Measurement is a costly non-value adding operation, and doing it twice is excess processing–wasteful. One reason I’ve been told is this is done to confirm the data provided by the supplier. But confirmation is possible only if the measurement process used by the customer matches the one used by the supplier.

Besides, if we are worried about the quality of supplier data, we should then focus efforts on deeply understanding their measurement process, monitoring its stability and capability, and working with the supplier to improve it if necessary. With that we should trust the measurement data the supplier provides and base our decisions on it, and eliminate the duplicate measurement step during receiving inspection.

[1] Eliminate Waste in Incoming Inspection: 10 ideas of where to look for waste in your process http://www.qualitymag.com/articles/92832-eliminate-waste-in-incoming-inspection Retrieved 2017-06-29


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