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 give us knowledge about what is going to happen – what will predictably occur next week, next day, next hour and so on.

What is predictable about customer demand, process performance, error rates and so on? What is predictable about individual performance, team performance or whole division performance? Measures of these things, plotted in capability 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 capability measures.

What governs efficiency or revenue?

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

Capability measures can be whole-process or whole-system measures.

The first step in choosing measures is to think about purpose.