Understanding the Causal Diagram
Six Sigma programs use the "fishbone" or "Ishikawa" cause and effect diagram for process analysis, while the 5-Whys method is used for root cause analysis. Although regarded as a simple tool, the 5-Whys method has proven to be very effective at isolating the core reason from among many possible causes.
The "Failure Mode and Effects Analysis (FMEA)" or the "Predictive Failure Analysis" methods on the other hand also make use of cause maps for risk analysis and theory testing. This method uses the analysis tree for mapping out the possible failures or defects and their effects.
Keep in mind the effectiveness of a cause and effect map is dependent on the information used to build the diagram. There are five attributes which should be present in any form of causal map:
- Parsimonious: In scientific principles, if there are two possible answers to a problem, only one of the two answers should be considered. The answer providing the simplest explanation is chosen rather than one requiring a more complex rationalization to support its theory.
- Complete: This means that in a cause and effect network scientific theory holds that all causal factors stem from a single root cause. The causal map should consider as symptoms only those that have complete links to the core reason.
- Precise: This denotes the quality of being exact and in strict adherence to the objectives of the diagram to arrive at a balanced conclusion and sound recommendation.
- Accurate: Although this attribute is synonymous with that of "precise," the greater concern is the concept of correctness that is not contestable as a stated fact.
- Visual: A reference to visual denotes that the links between nodes are presented with clarity; links should not cross even if similar nodes are exhibited close to one another.
Identify the Problem
The importance of identifying the problem accurately is in the same context as that of diagnosing an illness or health disorder to determine the most effective treatment.
Since the ultimate goal of a root cause analysis is the prevention of the occurrence or the recurrence of an unlikely event, the accuracy of the resulting origin and its symptoms is dependent on the problem stated.
In a 5-Whys cause map, the arrow points to the problem and not the other way around, since it is read and interpreted as "all causal factors and its origin, lead to the occurrence of the problem".
Analyze the Causes
Prior to this exercise, answers to a structured set of questions are gathered and evaluated as possible causes for a specific problem. They are then presented in the cause map as answers to the “Why” question.
In the image at the left, study the progression of the cause map. Each event is linked to the problem as it represents the answer to the why question.
If the effect was caused by two or more factors, it is quite important to still present them as a linear cause and effect relationship. The next image illustrates this particular step.
The exact number of why questions that an individual asks in an actual analysis of causal factors can be arbitrary.
Posing the questions should not be limited to five but the questioning should persist until it reaches a point in which asking the question why is no longer necessary or sensible. This denotes therefore, that the last answer presented in the cause and effect diagram represents the root cause of the problem.
Case Study Example
Case Study: A customer filed a complaint about a brand new washing machine, which conked-out during its initial use of barely a week since purchase. The service technician found a blown fuse; hence, his initial course of action was to replace the fuse. Unfortunately, he did not succeed in making the washing machine operational beyond the start-up process.
Faced with an irate customer, the technician decided to call the main office and asked for other possible solutions. However, it appeared that there had been several complaints from various customers in the area where that particular model was delivered. He was instructed to inform the client that the company would deliver a new unit that was of higher quality and more expensive at no additional cost.
All the defective machines were retrieved and carefully studied. Technicians couldn’t understand why the inspected motors overheated and resulted in blown fuses. In addition, they found damaged shafts and bearings, which they presumed were due to the overheated motor. Several reasons were considered as the cause of overheating: (1) the motor was misaligned and improperly installed (2) the cooling fan was inadequate (3) there was a loose fly-wheel (4) the shaft was damaged as a result of the overheating.
Further testing revealed:
- The cooling fan was eliminated after testing results showed that the other machines were good working conditions using the same cooling system.
- All overheated motors were inspected; none had a loose fly-wheel.
- New motors were installed in the defective washing machines and were double-checked for proper installation. Again, the motor overheated, which resulted in an improper transmission of electrical current from the motor to the machine being driven.
Create the Analysis Map
A root cause analysis was performed to determine the root cause of the machine's motor overheating.
Problem: The motors installed in the new batch of washing machine model WM3819-11 overheat.
Because the bearings connected to the shaft were not well-lubricated.
There was not enough lubricant reaching the bearings connected to the shaft.
There was no oil seal in each of the motors.
It was omitted as a component in the second batch of motors produced.
The oil seal was not stated as a component in the instruction manual for Washing Machine Model WM3819-11
The manual was not checked against the original manuscript or it was never proofread before it was printed.
At this point, further mapping for root cause analysis ends since posing more "why" questions no longer produces helpful answers. Proofreading is a basic procedure, which should not have been overlooked or by-passed; hence, the omission of this procedure was established as the root cause. Had the manual been properly proofed against the original, then the department responsible for installation of motor for that model would have complied with the stated requirements.
The immediate solution to prevent further consumer complaints is to recall the washing machine model and then install oil seals in their motors. All manuals for the motor should be corrected and proofread after the revisions. The unit assigned to produce the motor should be given a crash course in basic mechanical systems to ensure quality checking during production stage. Total reliance on manuals indicates lack of basic know-how and application of human judgement. The head of the production unit is held responsible for ensuring that all the manuals released for the production department are accurate.
- Cause Map examples for walk-through and case study discussions were created by the author for this article.
- Image: Fire Triangle Causal Relationships by Lthamilton / Wikimedia CCA-SA
- By Scavarda, Annibal José – Department of Industrial Engineering, Pontifical
Catholic University of Rio de Janei;Bouzdine-Chameeva, Tatiana Head of the
Department Information, Decision and Management, Bordeaux Business School
Goldstein; Susan Meyer – Curtis L. Carlson School of Management; Hays, Julie
M.– College of Business, University of St. Thomas; Hill, Arthur V.–
Curtis L. Carlson School of Management, University of Minnesota University
of Minnesota — “A Review of the Causal Mapping Practice and Research Literature” — http://www.qa.au.edu/page2/research/BSCCausalMappingMethodology.pdf