Root Cause Analysis
Root Cause Analysis (RCA) is the heart of all problem-solving methodologies such as 8D (complaint handling), DMAIC (more complex product or process problems), SCRA (Symptom – Cause – Remedy -Action for non-complex problems on the shop floor).
All methodologies, whether they consist of 4,5,8 or 20 steps, have in common the philosophy that the root cause of a problem must be found to avoid that the proposed solutions are merely symptom control.
Besides the two traditional tools for RCA (1,2), two additional ones (3,4) have proven their usefulness.
The 5 Why’s
Ask the question “Why?” 5 times to find the underlying root cause. The 5 Why’s is applicable to shop floor related relatively small problems in combination with organisational problems. For more difficult technical, process or product related problems, one can usually not get further than the first Why and the root cause cannot be found. When this is the case, the IS-IS NOT tool can be used (see below).
Cause and effect diagram
A brainstorming session is organised for possible root causes, which then are classified in groups, e.g. Man, Machine, Material or Method. The root causes are displayed in a so-called Fishbone diagram or Ishikawa diagram. Due to its ease of application, this tool has proven its usefulness and it is certainly applicable in an environment with little information on a process, e.g. when developing a new product/process.
The limitation of the Ishikawa diagram is that
- The proposed root causes are based on opinions rather than on facts
- Interactions between root causes are difficult to raise and to visualize
- Changes to processes are not covered
The outcome of this tool is a list of “potential” root causes. The selection of the most probable root cause(s) is done based on scoring, which is “problem solving by democracy”. It is Stanwick’s experience that in many cases the real root cause does not even appear in the Ishikawa diagram.
The IS-IS NOT
The IS-IS NOT tool (CH.Kepner, B. Tregoe, 1965) focuses on what facts the problem displays. “Let the problem and the data do the talking, not the engineers”.
Facts are collected about 4 dimensions: What, Where, When and the Extent the problem (“IS”), but also about What, Where, When and the Extent to which we would expect the problem to occur, but for one or another reason is not the case (“IS NOT”). By comparing the IS and IS NOT situations, one can find differences which, in combination with changes are the real root causes of the problem. Based on those facts it is possible to find mechanisms on why the problem appears like that, there, then and in that extent (IS), but also why the problem does not appear differently, elsewhere, on another time and not to another extent (IS NOT). Stanwick has applied this tool many times, sometimes even resulting in the surprisingly fast (< 2 hours) discovery of the root cause to problems which have been dragging for days, weeks or months.
Since the emergence of the 6-sigma methodology, the application of statistical tools to find root causes of problems has gained enormous interest. Some of these tools are 2-sample-t, Anova, Regression, Design of Experiments, Principal Components Analysis …