The quality of a product or process may be assessed by means of

- Variables : actual values measured on a continuous scale e.g. length, weight, strength, resistance, etc

- Attributes : discrete data that come from classifying units (accept/reject) or from counting the number of defects on a unit

**If the quality characteristic is measurable**

- monitor its mean value and variability (range or standard deviation)

**If the quality characteristic is not measurable**

- monitor the fraction (or number) of defectives
- monitor the number of defects

**Defectives vs Defects**

Defective or Nonconforming Unit

- a unit of product that does not satisfy one or more of the specifications for the product

–e.g. a scratched media, a cracked casing, a failed PCBA

Defect or Nonconformity

- a specific point at which a specification is not satisfied

–e.g. a scratch, a crack, a defective IC

**Shewhart Control Charts – Overview**

**Shewhart Control Charts for Variables**