Variation
No process gives the same result twice. Learning to tell everyday wobble from a real change is a quiet superpower.
About 5 minutes to read.
Key idea
Catch the same bus every morning and it never arrives at exactly the same minute. Most days it is within a few minutes of the timetable, and occasionally roadworks make it twenty minutes late. The everyday spread comes from many small causes, traffic lights, passengers, weather, and is called common cause variation. The roadworks are a special cause: something specific, findable and usually fixable.
Treating every wobble as if it were special is exhausting and makes things worse. A manager who praises the team on a good day and scolds them on a bad one, when both days are ordinary variation, teaches people nothing except to fear the numbers. Ignoring a real special cause is just as costly.
Customers feel variation more than averages. A delivery that usually takes two days but sometimes takes nine is worse for planning than one that reliably takes three.
Example: delivery times
The slow tail
A parts supplier proudly reports an average delivery time of two days. The workshops it serves are unhappy, and the figures show why: one delivery in ten takes over a week. Mechanics hold extra stock to protect themselves against the slow ones, and the supplier never sees the cost.
Reducing the spread, by finding why the slow deliveries are slow, matters more to these customers than shaving the average.