Probable Cause

Using probability to get to the bottom of the issue

by Moreno Broccoletti

A common challenge quality department staff face involves root cause analysis (RCA).

While tried-and-true techniques to conduct RCA—such as five whys or fishbone diagrams—are effective, there are limitations. Seasoned practitioners tend to identify root causes quicker than inexperienced workers, even when using the same tools. All experienced practitioners know there are some critical situations that facilitate the appearance of defects, even if this knowledge is not formally documented. The flowchart in Figure 1 standardizes the unwritten rules that ensure successful RCA.

Figure 1

Before using traditional RCA methods, the experienced practitioner defines the most appropriate investigative approach and determines where the defect could have originated by answering two key questions:

  1. Are we analyzing a new product?
  2. Are we analyzing a product from a new supplier?

Possible causes responsible for a defect have different probabilities of occurrence. Probability depends on whether it’s a new product or one that has been in production for some time. It also is correlated to whether a new supplier furnished the product. Fishbone diagrams do not take these pertinent elements into account. The idea is to give a weight, based on the probability of occurrence, to the possible causes of defects.

The flowchart in Figure 1 shows the stages of analysis that help an expert find the origin of a problem. As he or she addresses the questions in the diagram, all potential root causes are identified and can be fixed.

The last step shown in Figure 1 is improvement verification, which leads the practitioner to the end of the analysis if the problem is solved or down a new path in the investigation if it persists.

Facts and circumstance

Consider a scenario in which you encountered a part with a dimension that’s out of spec. If the part is from the usual supplier, which has already produced many good lots, you can assume the design is good, all specifications were communicated and the process flows with precision. In this case, we would first focus on unexpected changes on the supplier’s side.

If the part was provided by a new supplier, we must establish that all the specifications were updated and communicated before investigating for a supplier error. Finally, in the case of a new part, you must verify that the designed process and the equipment used was capable of producing a conforming product.

The use of the flowchart in Figure 1 accelerates the identification of root causes, and it levels the differences between inexperienced and seasoned operators. Causes are weeded out because they are arranged according to the probability of occurrence. The advantages of its use are even more evident in cases of multiple root causes. In these situations, the verification cycle begins with the last step previously taken into account.

Moreno Broccoletti is a quality engineer at ISA in Bastia Umbra, Italy. He earned a master’s degree in mechanical engineering from the University of Perugia in Italy.

Using FTA is a good technique to be adopted. The issue is the probility values are not easily obtained.
--Edward Chooi, 11-03-2015

to correlate probability of occurence to the cause / root cause to "identify" root cause is a good idea

Aylin N. Sener
Aylin N. Sener
--Aylin N. Sener, 04-28-2015

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