John Douglass and Shirley Coleman presented a case study of the application of a Taguchi experimental design at industrial Statistics in Action 2000 (Douglass and Coleman, 2000, pp. 11-18). The designed experiment was employed in a quality improvement project that had the aim of improving the yield at final test of breathing apparatus demand valves manufactured by Draeger Safety UK Ltd., Northumberland, UK. One response of interest was static pressure and the factors (and levels) were as follows:
1. demand valve type (standard/US version);
2. lever spring tension (standard/modified);
3. seal diameter (9.51 mm/9.63 mm);
4. bore diameter (9.16 mm/9.18 mm);
5. lever height setting (low/high);
6. diaphragm resistance (low/high)
The specifications tolerance for static pressure was 2.7 to 3.5 bar and at tomes reject levels approached 20% for static pressure, with the ‘great majority of these rejects exceeding the upper specification limit’.