Six Sigma Black Belt Exam Prep

Answer: A - An R-chart must have at least 20 subgroups of observed values to judge whether a process is in control or not.

Key Takeaway: An R-chart contains data collected from a business or industrial process at regular intervals. It measures the sample range over time which is a measure of dispersion. An R-chart must have at least 20 subgroups of observed values with each subgroup containing 3 to 6 observations. This minimum number of observations allows a manager to observe whether the variation falls within the upper and lower control limits.