Trees and roots

Q: How can fault tree analysis be used to improve a root cause analysis process?

A: Root cause analysis (RCA) is a term used to describe various methods, tools or philosophies aimed at solving and eliminating the recurrence of a problem. The concept is based on the fact that the most readily apparent, obvious solutions may just relieve symptoms and not address the real issue. The RCA process has the following steps, all supported by data and facts:

  1. Define the problem.
  2. Generate potential causes.
  3. Identify the true cause(s).
  4. Generate
  5. Implement the best solution(s).
  6. Monitor the effectiveness of the solutions.

The cause identification portion may benefit from the use of fault tree analysis (FTA), a systems reliability and safety tool that provides a diagram containing failure logic in a cause-effect format. It identifies the paths and probabilities that contribute to a specific failure and can be qualitative or quantitative.

To construct a fault tree, a single "top event" represents the problem, failure or hazard. Causal relationships are linked with "and" and "or" gates. "Or" gates represent individual events that could cause the linked event independently. "And" gates represent events that must happen in concert to cause the linked event. The fact that RCA is a problem-solving technique or failure-analysis tool implies the problem or failure has already happened, and now we’re fixing it. With FTA, there is an opportunity to use information that has been previously obtained.

Like any other quality or reliability tool, fault trees are most valuable when they are done proactively to manage risk and prevent failures, preferably early in product or process design. Risk management personnel can use FTA in conjunction with failure mode effects analysis (FMEA). The more important or complicated failure modes from the FMEA can be analyzed in further detail with a fault tree.

Ultimately, FTA can be an input to the identification and implementation of quality system, design, manufacturing and user controls. Philosophically, a control plan is actually the implementation of solutions before there are failures, a profound and powerful quality concept.

Certain RCA tools, if used alone, may miss the real root cause. For example, brainstorming and fishbone (Ishikawa) diagrams can be used with divergent thinking to list all possibilities, but convergence to the true root cause may be based solely on multivoting by team members—a process by which a large list of possibilities is narrowed to a smaller list of the top priorities.

If there are limited data or the team’s collective judgment is mistaken, solutions that address the wrong cause may be implemented. Then, RCA becomes more of a trial and error—or iterative—process. Fault trees can be quantitative, with calculations to determine the most likely foreseeable problems, to provide direction to the identification of the most likely cause.

Some RCA tools, such as the five whys, are linear in that they lead to one primary root cause. In complex situations, there could be more than one simultaneous root cause that applies to a particular failure mode. Consider a medical device for which design, manufacturing, labeling, intentional or unintentional user error, and patient characteristics all may interact (see Figure 1). A purely linear approach to RCA will not pick up these interactions. Branched causal relationships should be established. Fault trees with these types of branches contain several different five whys paths.

Figure 1

Figure 1 is a simplified fault tree for the potential hazard of patient infection related to the installation of a medical device. In practice, more detail could be built into the diagram. Actual probabilities for each type of failure could also be used if available.

Although traditional RCA tools can be used to obtain useful information quickly, RCA can become more time consuming than necessary if, at first, an incorrect or incomplete root cause is identified. Having an FTA available when RCA is performed can save time because the failure analysis team can capitalize on the prior work that identified probable problems and causes.

Scott A. Laman
Senior manager
Quality engineering and risk management
Teleflex Medical Inc.
Reading, PA

For more information

  • Rooney, James J., Lee N. Vanden Heuvel, Donald K. Lorenzo and Laura O. Jackson, "Cause and Effect," Quality Progress, February 2009, pp. 38-44.

Fine fellows

Q: I’m an ASQ fellow. How can I promote this and what it represents to company management?

A: Perhaps your company’s management would better understand what an achievement ASQ fellowship is if it knew all that goes into earning the title.

Being elected an ASQ fellow speaks to individual achievements and contributions to the quality profession. To be a fellow, you must be in the quality profession for at least 15 years, be endorsed by senior leadership of an ASQ member unit and meet minimum score requirements across the following six professional proficiencies:

  1. Technical experience: Covers technical knowledge and level of experience applying the use of concepts and defining actual results.
  2. Occupational responsibility: Growth of responsibility in the quality arena.
  3. Publications, papers and presentations: sharing of quality-related knowledge over an extended period of time.
  4. ASQ activities: actively involved in ASQ within a section, division or globally.
  5. Non-ASQ activities: participation in another professional organization with an explicit focus on quality-related methods or statistics in its stated purpose.
  6. Teaching and consulting: the sharing or teaching of quality principles to others.

It is an honor to be nominated by peers and elected a fellow by ASQ’s board of directors. Fellow members represent the upper echelon of the quality profession and serve as the backbone of ASQ.

Fellow status has been a part of ASQ since it was first founded in 1946. It was somewhat easier to attain at that time, however, because a senior member could simply petition for an upgrade. If the board of directors unanimously approved, that member would be recognized as a fellow. In 1948, there were 43 fellows; by 1956, the number increased to 218; by 1988, there were 472; today, there are 612.

Karen Prosser
Membership program administrator

For more information

Average Rating


Out of 0 Ratings
Rate this article

Add Comments

View comments
Comments FAQ

Featured advertisers