Pizza Delivery Example

Imagine there are two companies delivering pizzas in a city. Their average delivery times (in minutes) are seen in the table. Say, the upper specification limit is 30. That is the pizza has to be delivered within 30 minutes no matter where the client resides (within the city). (This limit is self-defined by the pizza outlets)

The Average for both the outlets is 20. But, as can be perceived by looking at the values, Outlet A seems to be more consistent, and shows less variability in cooking & delivering pizzas compared to Outlet B. Mean therefore is not a proper measure for comparing variations. A better way is thru standard deviation.

Instead of comparing process variability thru Mean, compare the sigma levels, which give a better insight into the process variability.

For Outlet B to better its process performance, it can target these: 1. Mean. 2. Standard Deviation 3. Sigma Level. Note that the specification limits cannot be changed as they are derived from customer expectations. By focusing on internal processes that are responsible for delays (such as cooking time, time lapse in dished out pizza and its pick up for delivery, etc.), Outlet B can improve on variability in delivery time.

Sigma Level = Diff bet'n mean & spec limit / Sigma

Note: In this example, the Simga Level we get is Zlt. To get Zst, add 1.5. So, as per Zst, the process of outlet A is at (7.91+1.5), and that of outlet B is (1.41+1.5).