Genichi Taguchi – Quality Loss Function and Robust Design

Born 1924

Genichi Taguchi is a Japanese quality expert, known for the Quality Loss Function and for methodologies to optimise quality at the design stage – “robust design“. Taguchi received formal recognition for his work including Deming Prizes and Awards.

Genichi Taguchi considers quality loss all the way through to the customer, including cost of scrap, rework, downtime, warranty claims and ultimately reduced market share.

Genichi Taguchi’s Quality Loss Function

The Quality Loss Function gives a financial value for customers’ increasing dissatisfaction as the product performance goes below the desired target performance.

Equally, it gives a financial value for increasing costs as product performance goes above the desired target performance. Determining the target performance is an educated guess, often based on customer surveys and feedback.

The quality loss function allows financial decisions to be made at the design stage regarding the cost of achieving the target performance.

Quality through Robust Design Methodology

Taguchi methods emphasised quality through robust design, not quality through inspection. Taguchi breaks the design process into three stages:

  1. System design – involves creating a working prototype
  2. Parameter design – involves experimenting to find which factors influence product performance most
  3. Tolerance design – involves setting tight tolerance limits for the critical factors and looser tolerance limits for less important factors.

Taguchi’s Robust Design methodologies allow the designer through experiments to determine which factors most affect product performance and which factors are unimportant.

The designer can focus on reducing variation on the important or critical factors. Unimportant or uncontrollable “noise” factors have negligible impact on the product performance and can be ignored.

Robust Design of Cookies

This is easier explained by example. If your business makes cookies from raw ingredients, there are many possible factors that could influence the quality of the cookie – amount of flour, number of eggs, temperature of butter, heat of oven, cooking time, baking tray material etc.

With Genichi Taguchi’s Robust Design methodologies you would set up experiments that would test a range of combinations of factors – for example, high and low oven temperature, with long and short cooking time, 1 or 2 eggs, etc. The cookies resulting from each of these trials would be assessed for quality.

A statistical analysis of results would tell you which are the most important factors, for example oven temperature affects cookie quality more than the number of eggs.

With this knowledge you would design a process that ensures the oven maintains the optimal temperature and you would be able to consistently produce good cookies.

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