Six Sigma Green Belt Certification Practice Exam

While reviewing an X-bar chart, you notice a set of seven consecutively decreasing points. What could have caused this behavior?

1. Process is over-controlled

2. Machine deterioration

3. New machine operator

4. Incorrectly calculated control limits

The correct answer is: Machine deterioration

The observation of seven consecutively decreasing points on an X-bar chart strongly suggests a non-random pattern in the data, which could be indicative of a systematic issue impacting the process. Machine deterioration is a plausible cause in this scenario because it can lead to a consistent decrease in the quality or output of a process. As a machine begins to wear out or malfunction, it may produce lower measurements consistently over time, leading to a downward trend in the data points plotted on the \bar{X} chart. When considering this information in the context of the other options, it's clear that they do not explain the specific trend observed as effectively as machine deterioration. Options related to process over-control or an incorrectly calculated control limit might lead to non-standard behavior, but they wouldn't necessarily result in a clear downward trend. Similarly, a new machine operator could introduce variability or changes, but that influence is less likely to produce a consistently decreasing trend without other contributing factors. Overall, new machine wear and the impact it has on performance explain the systematic decrease in the data points, making machine deterioration the most fitting reason for the observed behavior in the X-bar chart.