X-bar and s (XbarS) charts. These charts use variable data where the sample
size is large, usually greater than or equal to 10.
So, as you can see, you can sometimes use several of the charts, based on types
of data and on the size of the sample, and the size of the sample may change. While
some folks will quote hard and fast rules for the use of these charts, we have found
that organizations often modify when they are used and how they are used, based
on the preferences of someone influential in the organization. In any case, when
n
n
n
n
n
n
n
Statistical Process Control n 247
using these charts, look for trends or patterns in the data collected. Try to collect 20
to 25 groups of samples to be statistically correct, although 5 or 6 may prove useful
in detecting initial trends.
The control chart definitions use the terms attribute data and variable data.
Attribute data are data counted as discrete events or occurrences. For example, yes/
no, good/bad, is/is not defective. These data are usually counts of something.
Pages:
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528