Vol. 10, No. 8

August 2008

PQ Systems

A picture is worth...

Quality Quiz: With a video!

Data in everyday life

MSA with Jackie Graham

Bytes and pieces

FYI: Current releases


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Quality Quiz from Professor Cleary

Dr. Bickring thinks she knows it all. Does she?

After you have read the question, click on the response that you think is correct. If you answer correctly, you can register to win a free copy of Quality Gamebox.

Dr. Constance Bickring is confident of her ability to instruct the ER nursing staff in the fundamentals of statistical process control (SPC), in spite of the fact that she understands only the use of X-bar and R charts and little else in the way of statistical applications. In any case, she knew that the staff would not know as much as she did, so that was enough. At least, that had always been enough, when it came to proving her knowledge in a variety of areas. She recalled with pride her experience returning from Africa, where she had attended an international seminar on rabies. For months, she was considered to be a foremost expert on the disease even though she had never treated a patient with exposure to the disease.

In the seminar where Dr. Bickring had learned about charting, the instructor provided an example using 25 samples of 5 each, so this is the approach she uses in her own instruction of the nursing staff. She loves to use the sigma symbol, and is deeply involved in deriving the standard deviation when one of the nurses, Val Cano, asks why she has selected a sample size of 5. “Why not 6 or 7, or even 10? “ she wonders. Val had in fact also once participated in a seminar that offered an overview of SPC, and the instructor had used a sample size of 5 in that case as well.

Dr. Bickring had a ready answer, asserting that the selection of 5 was required for statistical precision, obliquely suggesting that any other sample size would be flawed in its statistical calculations. “Trust me,” she said, “this is statistically sound.”

Why is using a sample size of 5 conventional practice?

A) Dr. Bickring is right; it is grounded in statistical accuracy.

B) Using a sample size of 5 is the most economical choice.

C) A sample size of 5 renders the arithmetical calculations more convenient.

D) A sample size of at least five creates validity, but there is no magic in 5 itself.


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