The agreed that whenever the equipment was down, one team the team member would assume responsibility for filling out the check sheet. Note that the major causes of controller downtime identified on the cause-and-effect diagram were used to struc- ture the headings and subheadings on the check sheet. The team agreed that data would be collected over a four- to six-week period. As more reliable data concerning the causes of controller downtime became avail- able, the team was able to analyze it using other SPC techniques. Figure 5.25 presents the Pareto analysis of the controller failure data produced during the six-week study of the process. Note that concentration variation is a major cause of downtime. Actually, the sit- uation is probably more complex than it appears. The third largest category of downtime causes is reagent replenishment. Frequently, the reagent in the colorimeter on the con- troller is replenished because concentration has varied so far outside the process specifi- cations that reagent replenishment and colorimeter recalibration is the only step that can be used to bring the process back on line. Therefore, it is possible that up to 50% of the downtime associated with controller failures can be attributed to concentration variation.