What are Fishbone (Ishikawa) Diagrams, and how can they be used with inspection data?

2014-09-03

We like Ishikawa's Fishbone Diagrams, also referred to as cause and effect diagrams for good reason. They are great for figuring out why something isn't working. Karou Ishikawa created the Fishbone Diagram in 1968. "Common uses are product design and quality defect prevention, and to identify potential factors causing an overall effect. Each cause or reason for imperfection is a source of variation. Causes are usually grouped into major categories to identify these sources of variation." For our clients, it is not uncommon for inspections to uncover trends in failures. When this happens, it is natural to want to know why the problem is occurring so appropriate corrective actions can be taken. An example is listed below. 1. We discussed the problem and defined it in a way that was specific and relevant. In this case, there were a series of rejected pre-shipment inspections where paint defects were exceeding AQLs as part of the visual evaluation. 2. We brainstormed categories that would have an effect on this problem. We read about and do find the use of sticky notes to be a very effective way to organize this information. Also, looking through examples can help with this, as there are a handful of very common categories used. The categories are used as the branches off the main arrow. 3. We brainstormed the issues digging deeper into each one and included them where they fit best in the categories. Keep asking why to get a more in-depth evaluation. Layers in the branches can subdivide out the issues further, as necessary. The general rule is to ask "why" five times. 4. We analyzed the diagram and did further research into the causes we listed. When identifying causes and incorporating corrective action, follow-up metrics are useful in determining if your actions have produced the desired result of effectively resolving the problem you originally stated. See a larger version of the example diagram here: http://blog.proqc.com/wp-content/uploads/2014/01/Fishbone_Example_Defect-copy1.jpg A brief video explaining the Fishbone Diagram is here: http://blog.proqc.com/short-and-simple-the-fishbone-diagram-explained

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