Defect Root-Cause Analysis: Where do you start?

Manufacturing companies across the world use Non-Conforming Material Reports (NCMRs) or Discrepant Material Reports (DMRs) to identify and track parts that do not fit or function as desired.

The Current Method

  • MyCo Caused (replace MyCo with your company name)
  • Supplier Caused

In most cases, this is the first selection a technician makes. This is surprising given the limited amount of information available at the point in time when the defect is first discovered. The technician is forced to make a best guess, and checks off one of the two boxes above.

As any supplier quality engineer will tell you, over half of the problems classified as "supplier caused" are typically reversed after a detailed root-cause analysis. To make things worse, since the process begins by assigning blame, it sets up a confrontation between supplier and customer, making it difficult to objectively solve the problem. Over time suppliers grow immune to the frequent erroneous accusations, and stop treating quality issues with the required level of seriousness.

So instead of beginning by assigning blame, I am going to propose a more objective preliminary classification for defects.

A Proposed Method

Before we begin, let's remind ourselves of our goal: Our goal is to ensure that this defect - or better yet, this type of defect - never occurs again! So always begin by reviewing the spec and the defective part together. This includes reviewing drawings, bills of material, and requirements documents, and either locating data for, or verifying calibration and repeating measurements on, the necessary part characteristics.

Next, classify the defect into two very high-level categories:

  • Inadequate Process: Part does NOT meet specifications, OR
  • Inadequate Specifications: Part meets specifications, but the specifications themselves are inadequate.

The defect root-cause tree below provides some examples about what types of defects are classified into each category:

Defect Root Cause Tree - 1factory

This classification works whether a defect is found in manufacturing or in the field, and will help you initiate root-cause analysis in a more collaborative manner.

Remember to always:

  • Treat every defect seriously. Ensure each defect is recorded and reported. The availability of frequency data will help you to rank and prioritize your actions.
  • Record measurements. Require all your suppliers to record and provide actual part measurement data. The availability of specification-level measurement data will speed-up your root-cause analysis.
  • Make no assumptions about the accuracy of the spec itself. You will be surprised at how often your specifications are incomplete, inadequate, or simply incorrect.