Chapter 17: Quality Control and SPC

Quality Control

to ensure that a good or service conforms to specifications and meets customer requirements by monitoring and measuring processes and making any necessary adjustments to maintain a specified level of performance.

Three components to any control system

1. A performance standard or goal.

2. A means of measuring actual performance.

3. Comparison of actual performance with the standard to form the basis for corrective action.

1:10:100 Rule

If a defect or service error is identified and corrected at the design stage, it might cost $1 to fix. If it is first detected during the production process, it might cost $10 to fix. However, if the defect is not discovered until it reaches the customer, it might cost $100 to correct.

Quality at the source

the people responsible for the work control the quality of their processes by identifying and correcting any defects or errors when they first are recognized or occur

Supplier certification and management

If incoming materials are of poor quality, then the final manufactured good will certainly be no better.

In-process control

needed to ensure that defective outputs do not leave the process and, more important, to prevent them in the first place.

Finished-goods control

focused on verifying that the product meets customer requirements.

Net Promoter Score (NPS)

developed by (and is a registered trademark of) Fred Reichheld, Bain & Company, and Satmetrix, to measure customer loyalty.

Reasons variation occurs

inconsistencies in material inputs, changes in environmental conditions, machine maintenance cycles, customer participation and self-service, tool wear, and human fatigue.

Common cause variation

the result of complex interactions of variations in materials, tools, machines, information, workers, and the environment.

Special (or assignable) cause variation

arises from external sources that are not inherent in the process, appear sporadically, and disrupt the random pattern of common causes. Occurs sporadically and can be prevented.

In control

If no special causes affect the output of a process

Out of control

when special causes are present

Statistical process control (SPC)

a methodology for monitoring the quality of manufacturing and service-delivery processes to help identify and eliminate unwanted causes of variation.

Control chart

a run chart to which two horizontal lines, called control limits are added: the upper control limit (UCL) and lower control limit (LCL)

Continuous metric

one that is calculated from data that are measured as the degree of conformance to a specification on some continuous scale of measurement.

Discrete metric

one that is calculated from data that are counted

“x-bar chart”

used to monitor the centering of the process

R-chart

used to monitor the variation in the process.

P-chart

monitors the proportion of nonconforming items. Often, it is also called a fraction nonconforming or fraction defective chart.

C-chart

monitors the total number of nonconformances per unit when the size of the sampling unit or number of opportunities for errors is constant.

Two key issues that designing control charts involves

1. Sample size

2. Sampling frequency

Process capability

refers to the natural variation in a process that results from common causes

Process capability study

a carefully planned study designed to yield specific information about the performance of a process under specified operating conditions

Typical questions that are asked in a process capability study

1. Where is the process centered?

2. How much variability exists in the process?

3. Is the performance relative to specifications acceptable?

4. What proportion of output will be expected to meet specifications?

Process Capability Index

The relationship between the natural variation and specifications is often quantified by a measure