CSSBB Primer Sample Questions – Need Help

[Lori Snetsinger]

Hello there,

I am currently studying for my CSSBB exam, and I am having some trouble with the Primer practice questions in section 7, primarily #s 7 and 33. Question 7 reads: The report Cpk for a process with an average of 28, a spread of 10 units, and USL and LSL of 35 and 15, respectively would be:

a. 1.6
b. 1.4
c. 1.8
d. .714

I have no idea how to estimate the standard deviation given this information, as “spread” isn’t covered anywhere in the Primer section. Please help!

[waardg]

Cpk = MIN [(USL – mu) / 3 sigma)] ; [(mu – LSL)/3 sigma)]
sigma = Rbar / d2
Rbar = 10, d2 comes from a table. (R= Range = Spread)

[Lori Snetsinger]

The problem is that I don’t have the value for n to be able to look up the d2 value.

[Ted Hessing]

Here’s a table I found, Lori. http://www.bessegato.com.br/UFJF/resources/table_of_control_chart_constants_old.pdf

[joey1202]

Which d2 should be used using this table since n is unknown?

[Lori Snetsinger]

I found an answer to this on the interwebs, here’s an excerpt, as well as the link:
In the question process spread is given 10 units… And you have taken it as standard deviation directly….
This process spread of 10 units is assumed to be 6 sigma. Thus sigma is 10/6.

http://www.cram.com/flashcards/ss-stats-formulas-equations-5453251

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