Re: xBar-R versus X-chart control charts

[GrantBlair@aol.com]

In a message dated 2/2/2000 1:08:27 AM Eastern Standard Time, 
Steven_S_Prevette@RL.gov writes:

<< 2.  A fixed subsample size (usually 4 or 5) is a more "rational" 
subgrouping
 than accumulating by calendar month.  For example, one month's data may be
 based upon 10 items, and the next month based upon 2.  This comes into play
 if you are calculating the average value (such as average cycle time) for
 the month. >>
Generally speaking, the range is considered a more efficient estimate for 
subgroup
sizes less than 9. For larger subgroup sizes (10 or more), sigma is better.
However, I gathered from your post you preferred to plot individuals data.
A useful (and less confusing to the layman}:-) alternative is to calculate 
sigma
using the Mean Square Successive Difference method. Subtract each current 
measurement from the previous, square this difference, sum all the squares,
divide by N-1,then take the square root. Many statistics programs have 
this computation as a "hidden" command (I know SAS and Minitab do).

GrantBlair@aol.com
Ninety Six SC
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