Question

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

Answer 1

We prepare the range chart that could plot ranges of various sample data; such range chart would be applicable when there are subgroup sizes over two and less than ten under each sample. Range is calculated as Maximum data value – minimum data value under each sample. We prepare range chart to understand how sample data’s ranges are in statistical control, we check for variability and we check what kind of variations are arrived in forming the control chart. Such variation could include assignable causes and common or random causes.

Standard deviation charts are used when subgroup sizes are more than ten as we could understand better spread of subgroup data under each sample when subgroup size under each sample increases over ten. Standard deviation is considered better measure to calculate the variation, hence, we will use such standard deviation charts in critical situation like number of subgroup increases above ten. To monitor spread of manufacturing processes, we could take many samples with keeping few subgroups, here, range chart would be good choice, as we don’t have large subgroup size and we could easily calculate range data, we could understand the variability in the process through statistical control charts preparation.