Statistical Quality Control (SQC) & Process Variability

Statistical Quality Control (SQC)

A quality control system using statistical techniques to control quality by performing inspection, testing & analysis to conclude whether the product is as stated or designed quality standard

- Relying on the probability theory, evaluates batch quality & controls the quality of processes or products

- Makes the inspection more reliable & less costly

SQC is used to describe the set of statistical tools used by quality professionals

• Descriptive statistics

- Used to describe quality characteristics & relationships

• Acceptance sampling

- Used to randomly inspect a batch of products to determine acceptance or rejection of entire lot based on the results

- Does not help to identify and catch the in-process problems

• Statistical process control (SPC)

- Involves inspecting the output from a process & identifying in-process variation

- Quality characteristics are measured & charted

Process Variability

Variation exists in all processes

Variation can be categorized as either

• Random variation (Natural variation in the output of a process, created by countless minor factors)

- Random causes that cannot be identified
- Unavoidable: inherent in the process
- Normal variation in process variables such as material, environment so on
- Can be eliminated only through improvements in the system

• Assignable variation (In process output, a variation whose cause can be identified. A nonrandom variation)

- Causes can be identified and eliminated
- Poor employee training, worn tool, machine needing repair
- Can be modified through operator or management actions