Why is quality concerned with reducing variation? Because it improves prediction. Managers should be more interested in what is about to happen than what has happened.

Capability measures tell us what is about to happen - what will predictably occur next week, next day, next hour and so on.

What is predictable about customer demand, process efficiency, error rates, time for work to go through a process? What is predictable about individual performance, team performance or whole division performance? Measures of these things, plotted in control charts, give managers and workers the means to learn from and improve performance.

For example, if we know what matters to our customers, we can measure and improve it using measures of variation.

What matters to customers - are there any 'key quality characteristics'? It is vital to know what is happening to the customer, especially on those things by which the customers judge you.

What governs efficiency or revenue?

It is equally vital to know how well processes perform, particularly those that affect costs and sales.