SPC - independent data

["Dr A D Burns" <mmedia@ozemail.com.au>]

In a message dated 7/2/2003 1:34:46 PM Eastern Daylight Time, David =
Kerrigde writes:

<<As we know, the commonly taught idea that observations have to be =
normally distributed before we can apply SPC is
wrong. But they do have to be independent.<<

The latter point is is not correct. Data need neither be normally =
distributed nor independent, to use control charts. Wheeler gives an =
extensive discussion on pages 278 to 288 of "Advanced Topics in SPC". =
Even with large degrees of autocorrelated data, Wheeler points out that =
although the control limits are effected (narrowed), the purpose of the =
analysis is insight (into the process), rather than the numbers (p283).

Dr Tony Burns
MicroMultimedia
www.q-skills.com
mmedia@ozemail.com.au