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 could result in Y. There could be multiple X’s. These are independent factors. 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, the formation of a thin sheet of iron undergoes several processes. The input variables are Iron Ore (Wrought Iron), charcoal, other chemicals and a furnace. This wrought iron is transformed through use of all of the input raw materials in the right proportion and heating in the furnace into a desired outcome. The raw materials and furnace is the X’s, the mixing of raw materials and heating are the transformation process function f, and the resulting Iron sheet is the Y. Ɛ can be the varying degree of temperature throughout the furnace resulting in non-uniform sheet of metal (errors in the process).
In the Pizza delivery example, some of the reasons for not meeting the pizza delivery time of 30 minutes could be Heavy Traffic, Delivery Driver did not know the delivery address, Pizza was not prepared in-time, among others. Thus, in this example, Delivery time of Pizza is “Y” and the reasons for not delivering the pizza on-time are “X’s”.
After understanding the two examples described above, in order to get results, should we focus on “Y” or “X”?
Focus the Causes (X) and NOT the Result (Y): Whenever you do a Six Sigma project, the focus of the project team needs to be on identifying of causes and mitigating them. The Result will automatically improve if the causes are dealt correctly. In the above example, formation of an iron sheet is the result and all the input materials are the causes. Thus, focus on the Causes and NOT the Result.