Quality in Design - Medical Device Design: Innovation from Concept to Market - page 147

Hence the probability of failure is

P(X) 1/1 /1 00

×× ,

00 1/1 00 1/1 000000 ,

This is called built-in redundancy. It is a technique used in the aircraft industry to design
robustness into a system.

7.5.1.1 Ishikawa Diagram
Although this is not strictly a fault tree analysis it is another example of a diagrammatic
structure that enables a root cause to be identified. The Ishikawa diagram is simple in format
but its strength lies in its simplicity. It is commonly called the herringbone diagram , because
it looks like…a herringbone!

Figure 7.13 illustrates a typical Ishikawa framework (Bicheno & Catherwood, 2005) . This
format has six main headings: measurements, materials, people, environment, methods, and
machines. The idea is to focus on these headings and determine how they may contribute to
the failure of your device. Taking each heading in turn:

l

Measurements: how data is used and obtained.

l

Materials: raw materials used; where they come from and who supplies them.

l

People: anyone involved with the making and use of the device and how they interact.

l

Machines: dependency of the device on any machines.

Measurements Materials People

Failure mode or defect

Environment Methods Machines

Figure 7.13
Ishikawa diagram.