November 2001

Vol. 3, No. 11

Quality Quiz from Dr. Cleary

You're right!

There are four types of data:
1. Nominal. The word 'nominal ' comes from a Latin word, meaning 'name. ' Nominal data allows for distinctions to be made between items, such as male or female; programmer or nonprogrammer; rainy or not rainy.
2. Ordinal data. The word ordinal comes from another Latin word, meaning 'order. ' Ordinal data identifies distinctions among items with respect to degree or order: e.g., small, medium, large; short, tall; or big, little.
3. Interval data. Interval data reflects a measurement of the difference between items.

$2,400 and $2,200 = difference $200

6 feet and 5 feet = one foot difference

10 days and 2 days = eight days difference

4. Ratio data. Distinguishes among items using a common denominator, that is, percentage.

My day class is 50 percent female.

My evening class is 30 percent female.

30 percent of part A are bad

20 percent of part B are bad

Raney has ratio (commonly known in SPC as attribute) data, rather than interval data. A p-chart or an np-chart would be appropriate to use in this case. Note: Under some circumstances, some would suggest that an X-MR chart is the right choice here. For more discussion, contact me at


Copyright 2001 PQ Systems.

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