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Gold/Mining/Energy : Naxos Resources (NAXOF)
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To: Kim W. Brasington who wrote (17227)12/13/1998 8:13:00 PM
From: Chuca Marsh  Read Replies (3) of 20681
 
Kim, is this the NWA that did the testing?:
nwasoft.com <<..
For chemicals producers, the problems begin with how the Cpk calculation is usually represented, requiring both upper and
lower specifications. Impurities usually have an upper specification ("not to exceed") but no lower specification (the goal is
usually zero). Many QC workers in the chemical industry will define a lower specification of zero in hopes of being able to use
the common equations for Cpk. The results can appear disastrous even in a process that is operating properly, since most of
the data is close to what has been defined as the lower specification.

In this example, a minerals producer has a product with a requirement of keeping Manganese Oxide (MnO2) below 2.5%.
The QC staff first set specifications of 0.0 to 2.5 and attempted a process capability analysis. The resulting histogram looked
improbable. With an unacceptable Cpk of 0.7389, the "normal" curve overlay (estimated from the data) extended below zero
where no actual analytical results are possible. (See Figure 4.)

Figure 4
Product impurities charted using normal process capability settings
show data below zero, where no actual analytical results are possible.
Unacceptable Cpk does not reflect actual process capability...>>
Chuca
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