Certified Production & Operations Manager (POM) Practice Exam

What is the usual purpose of an R-chart in statistical process control?

1. To identify trends in data

2. To signal whether there has been a gain or loss in dispersion

3. To monitor the average values over time

4. To assess the probability of defects

The correct answer is: To signal whether there has been a gain or loss in dispersion

The R-chart, or range chart, serves the specific purpose of monitoring the variability or dispersion of a process over time. It is particularly useful in statistical process control for determining whether the process is behaving consistently within its expected limits. When using an R-chart, data is collected in subgroups, and the range of each subgroup is calculated to assess variability. Option B is correct because the primary function of the R-chart is to signal whether there has been a gain or loss in dispersion. If the points on the R-chart indicate that the range is consistently moving outside of control limits, it suggests that the process variability is changing, prompting further investigation into possible causes. Other options do not correctly represent the primary purpose of the R-chart. Identifying trends in data is more aligned with the function of a X-bar chart, which focuses on central tendency. Monitoring average values over time is also aligned with X-bar charts rather than R-charts. Assessing the probability of defects typically falls under the domain of control charts for attributes, such as p-charts or np-charts. Thus, the R-chart distinctively targets monitoring dispersion, making option B the suitable answer.