Question

What is the range chart? Why were range charts rather than standard deviation charts once used to monitor
the spread of manufacturing processes?

Explain why the mean chart and the range chart must be examined together.

Answer 1

R chart in combination with the X chart or mean chart represents the dispersion of data that makes it easier for us to recognize any patterns that are present which becomes useful in determining if the process is moving towards being out of control before it actually occurs in the process. This is because unlike the standard deviation representing the average displacement the data shows away from the mean values, the fact that the R chart shows the spread both above and below the center line and makes it easier to identify any patterns that makes it more useful when used in combination with X chart. Another reason why an R chart is not used alone is the fact that the dispersion shown by the process might get out of the set limits, while the average mean range stays the same. This would theoretically imply that the process is getting out of control when it might not be true if the mean chart is producing the necessary output between the control limits. The same can be said for the X chart being out of control where the range is in control because the data moves relatively with the average, bringing the absolute range to be closer to the centerline, when in fact that process is producing out of control. Since we do not want to have a semi-understanding of our process, we will make use of both the charts in conjunction to monitor that both the average output and the range between the averages are within the specified control limits we have set.