Assignable Causes: The second categ

Assignable Causes: The second category of variation, assignable causes of variation, also known as special causes, includes any variation-causing factors that can be identified and eliminated. Assignable causes of variation include an employee needing training or a machine needing repair. Let us return to the example of the lab analysis process. Figure 3.6 shows how assignable causes can change the distribution of output for the analysis process. The green curve is the process distribution when only common causes of variation are present. The purple curves depict a change in the distribution because of assignable causes. In Figure 3.6(a), the purple curve indicates that the process took more time than planned in many of the cases, thereby increasing the average time of each analysis. In Figure 3.6(b), an increase in the variability of the time for each case affected the spread of the distribution. Finally, in Figure 3.6(c), the purple curve indicates that the process produced a preponderance of the tests in lessthan average time. Such a distribution is skewed, or no longer symmetric to the average value. A process is said to be in statistical control when the location, spread, or shape of its distribution does not change over time. After the process is in statistical control, managers use SPC procedures to detect the onset of assignable causes so that they can be addressed.