Inspection in action
Consider, for example, a manufacturing company that was using an industry standard 100% visual-inspection program to review
finished vials. The program had been in place for several years and results of the visual inspections for spots on stoppers
had typically been in the 2–5% range. However, over the course of several weeks, the cull rate escalated from 5% to more than
20%.
Alarmed, the quality unit conducted an investigation, but was unable to identify a clear reason for the dramatic increase.
The inspectors and environment had not changed. Puzzled, the company brought in a consultant to assist with the investigation.
The consultant pointed out a fundamental measurement rule and asked, "Where is the standard for spots on stoppers?" The reply
was, "There is none. Each inspector uses their best judgment." The consultant responded, "Without a standard, there is no
basis for measurement."
Essentially, each inspector was using his own floating scale without any reference point. As the inspection continued, one
inspector pointed out that he found a few more spots on the vials than usual. Sensitized to this information, the other inspectors
became more vigilant and began to find more spots themselves. As word spread, the inspectors became hypersensitive, finding
more and more spots as each lot was inspected until they were identifying more than 20% rejects.
Some identified spots were so small that the supervisors and managers could not see them even when pointed out by the inspectors.
Without a standard for reference, any perceptible dot became a defect. A team discussion led to a defined specification: a
spot below a certain size was not to be counted as a defect.
Summary
The company described in the above example soon adopted the TAPPT dirt estimation chart (1) as its reference standard. Each
operator was given an original card for use during inspection. If any questions arose, the inspectors could compare potential
defect spots to the chart and also review them with the supervisor.
Within one working shift, the defect level for spots on stoppers dropped back to the 2–5% range. The lesson learned: measurement
is a comparison to a reference standard.
Lynn D. Torbeck is a statistician at Torbeck and Assoc., 2000 Dempster Plaza, Evanston, IL 60202, tel. 847.424.1314, Lynn@Torbeck.org
,
http://www.torbeck.org/.
Reference
1. Dirt and Size Estimation Charts,
http://www.tappi.org/Standards-TIPs/Dirt-Size-Charts.aspx, accessed Aug. 19, 2010.
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