The concept Y=f(x) reflects the type of correlation between the inputs (x’s) and the output Y, which is the result of the process. The process itself is represented by f. This is a level-thinking approach that runs as a common thread through DMAIC. The formula is used when the output (Y) of a process (f) needs to be changed/improved, and the process inputs or causes (x’s) must be identified to achieve the desired state. In Six Sigma, measurements and analyses are conducted for this purpose. At the same time, the concept helps in identifying underlying root causes – also symbolized by x’s – that negatively impact Y. Y=f(x) thus serves as a foundation for process optimization.