Sqc - Statistical Quality Control

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Statistical quality control (SQC)
The application of statistical techniques to measure and evaluate the quality of a product, service, or process.

Two basic categories:
I. Statistical process control (SPC):

- the application of statistical techniques to determine whether a process is functioning as desired II. Acceptance Sampling:

- the application of statistical techniques to determine whether a population of items should be accepted or rejected based on inspection of a sample of those items. Quality Measurement: Attributes vs Variables Attributes:

Characteristics that are measured as either "acceptable" or "not acceptable", thus have only discrete, binary, or integer values. Variables:

Characteristics that are measured on a continuous scale.

Statistical Process Control (SPC) Methods
Statistical process control (SPC) monitors specified quality characteristics of a product or service so as: To detect whether the process has changed in a way that will affect product quality and To measure the current quality of products or services. Control is maintained through the use of control charts. The charts have upper and lower

control limits and the process is in control if sample measurements are between the limits. Control Charts for Attributes P Charts - measures proportion defective. C Charts - measures the number of defects/unit. Control Charts for Variables

X bar and R charts are used together - control a process by ensuring that the sample average and range remain within limits for both. Basic Procedure

1. An upper control limit (UCL) and a lower control limit (LCL) are set for the process. 2. A random sample of the product or service is taken, and the specified quality characteristic is measured. 3. If the average of the sample of the quality characteristic is higher than the upper control limit or lower than the lower control limit, the process is considered to be "out of control".

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