2 replies to this topic

[Sandeep Goyal]

Hi,

I want to understand the difference in usage or significance of both these charts apart from the fact that Range is involved in one of these and Sample SD is involved in the other

How do we really decide:

1. When these charts should really be applicable/useful  in practical scenario

2. Which of these should be preferred.

Please share your views on the same.

[Suresh Jayaram]

Dear Sandeep,

This depends on your subgroup size. If subgroup size is between 2 and 9, then you would use the Xbar-R chart and if the subgroup is greater than 9, then you would use the Xbar-S chart.

An Xbar-S chart is of course more accurate compared to the Xbar-R chart because you only consider the extreme points in an R chart to estimate the variation and ignore all the intermediate points. However, an Xbar-R chart is easier to draw by hand on the shop floor by the operator while you would need a calculator or computer to do the Xbar-S chart. There is power to having the operator do this by hand and understand what is happening rather than have a computer do it and no one looks at the computer for days or months!

SJ

[Sandeep Goyal]

Dear SJ,

Thanks a lot for the clarity on the same.

Sandeep