What is the Focus of Six Sigma?

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Focus of Six Sigma

Y is outcome(s) or result(s) you desire and need from a process. This is a dependent factor and it depends on the X’s.
X represents the input factors that may influence Y. There may be several X’s. They are explanatory or input variables; calling them independent variables does not mean that the inputs must be statistically unrelated.
Ɛ represents the presence of error, or uncertainty surrounding how accurately the X’s are transformed to create the outcome.

In other words, the input variable(s) are transformed by a function (or process) and combined with error to form the output. The Y results from, or is a function of the Xs. To determine a desired outcome, you apply a transformation process or function, f, on the inputs.

For example, producing a thin iron sheet involves several processes. Inputs can include iron ore, charcoal, other process chemicals and a furnace. Iron ore is not the same material as wrought iron: ore is processed into metal, and refining can produce wrought iron before further forming. In this simplified example, the materials and furnace conditions are X’s; using the right proportions, heating, refining and forming make up the transformation process f; and the finished iron sheet is Y. Unmeasured temperature differences within the furnace can contribute to ε and a nonuniform sheet. If temperature is measured and included in the model, it is one of the X’s.

In the pizza-delivery example, possible reasons for missing the 30-minute target include heavy traffic, a driver who does not know the address, or a pizza that is not prepared on time. Delivery time is Y; the relevant input conditions are X’s. Investigate these possible causes rather than assuming that each one explains every late delivery.

After understanding the two examples described above, in order to get results, should we focus on “Y” or “X”?

Focus on the Causes (X) and Verify the Result (Y): A Six Sigma team identifies and addresses the causes that influence its desired outcome. Improving the relevant X’s should improve Y, but the team still measures Y to confirm the result. In the iron-sheet example, the finished sheet is the result; the materials and process conditions are possible influences. Focus improvement work on the verified causes, and use the result to check whether the changes worked.