If a value assigned to a unit is non-numerical. it is described as

Qualitative data, often referred to as attribute data in quality control, describes characteristics or qualities that are non-numerical. This type of data is used for classification and categorization based on observation rather than measurement. For example, classifying a product as "conforming" or "nonconforming," identifying a surface finish as "smooth" or "rough," or noting the color of an item are all instances of assigning a qualitative, non-numerical value. It answers questions of "what kind" or "which category" an item belongs to.

A. nonconforming: This is a specific classification (a type of qualitative data), not the general term for all non-numerical data. A unit can have a non-numerical value (e.g., "blue") and still be conforming.
B. quantitative: This describes numerical data obtained by measurement (e.g., length, weight, temperature). It is the direct opposite of non-numerical data.
C. variable: This is a synonym for quantitative data, representing measurements on a continuous or discrete numerical scale.

1. Gryna

F. M.

Chua

R. C. H.

& DeFeo

J. A. (2017). Juran's Quality Planning and Analysis: From Product Development Through Use (6th ed.). McGraw-Hill Education. In Chapter 3

Section "Types of Data: Variables and Attributes

" it is stated: "Data that are countable are called attributes data... Data that are measurable are called variables data." This establishes the distinction between countable/categorical (qualitative) and measurable (quantitative) data.

2. Montgomery

D. C. (2019). Introduction to Statistical Quality Control (8th ed.). John Wiley & Sons. In Chapter 1

Section 1-3.1

"Descriptive Statistics

" the text distinguishes between data types: "Data can be either quantitative (numerical) or qualitative (categorical)... When the characteristic being studied is nonnumeric

it is called a qualitative or categorical characteristic."

3. ASQ Quality Press. (2017). The Certified Quality Engineer Handbook (4th ed.). In Section V.A.1

"Types of Data

" the text defines qualitative data as follows: "Qualitative data are data that can be classified into categories... Qualitative data are also known as attribute data" (p. 279).