American statistician W. Edwards Deming defined two origins of variation in a process or system. Common cause of variation is the noise in the system, constantly occurring and predictable by a measure of probability. Special cause of variation is one that is novel and unpredictable, one that is always a surprise – this is the signal in the system.
Deming wasn’t the first person to distinguish between the two – credit should also be given to his contemporary Walter Shewhart – but he’s as good an introduction to the concept as any.
In questions of management and organisation, it’s vital to distinguish between the two. Treating common causes, i.e. those arising in the system, as if they were due to individuals in your work force not only gives you no insight into the function of the system, but also runs the risk of demoralising your team. The tendency to do this is particulaly prevalent because experiments by Deming have shown that this type of variation is not evenly distributed among workers, because the system affects people in different ways. The fact that a member of your sales team sells on the lower end of a given range compared to a colleague is in as much as 94% of cases (Deming’s approximation) the product of the system.
Want to change the system? You can’t do it by forcing your workforce to work harder. This only breeds resentment. It is the responsibility of management to see how behaviour is affected by the system, and consider how the system can be changed to improve outcomes.
For more information on Deming’s ideas, try starting with this video on burnt toast. Never stop learning!