Statistical quality control (SQC)

Statistical quality control (SQC)

The general category of statistical tools used to evaluate organizational quality.

Descriptive statistics

Statistics used to describe quality characteristics and relationships.

Acceptance sampling

is the process of randomly inspecting a sample of goods and deciding whether to accept the entire lot based on the results. Acceptance sampling determines whether a batch of goods should be accepted or rejected.

Common causes of variation

Random causes that cannot be identified.

Assignable causes of variation

Causes that can be identified and eliminated.

Mean (average)

A statistic that measures the central tendency of a set of data.

Range

The difference between the largest and smallest observations in a set of data.

Standard deviation

A statistic that measures the amount of data dispersion around the mean.

Out of control

The situation in which a plot of data falls outside preset control limits.

Variable

A product characteristic that can be measured and has a continuum of values (e.g., height, weight, or volume).

Attribute

A product characteristic that has a discrete value and can be counted.

x-bar chart

A control chart used to monitor changes in the mean value of a process.

Range (R) chart

A control chart that monitors changes in the dispersion or variability of a process.

P-charts:

A control chart that monitors the proportion of defects in a sample.

Used when observations are placed in either of two groups.

Examples:

Defective or not defective

Good or bad

Broken or not broken

C-charts

A control chart used to monitor the number of defects per unit.

Used when defects can be counted per unit of measure.

Examples:

Number of dents per item

Number of complaints per unit of time (e.g., hour, month, year)

Number of tears per unit of area (e.g., square foot, square meter)

Process capability

The ability of a production process to meet or exceed preset specifications.

Product specifications

Preset ranges of acceptable quality characteristics.

Process capability index

An index used to measure process capability.

Six Sigma quality

A high level of quality associated with approximately 3.4 defective parts per million.

Sampling plan

A plan for acceptance sampling that precisely specifies the parameters of the sampling process and the acceptance/rejection criteria.

Operating characteristic (OC) curve

A graph that shows the probability or chance of accepting a lot given various proportions of defects in the lot.

Acceptable quality level (AQL)

The small percentage of defects that consumers are willing to accept.

Lot tolerance percent defective (LTPD)

The upper limit of the percentage of defective items consumers are willing to tolerate.

Consumer’s risk

The chance of accepting a lot that contains a greater number of defects than the LTPD limit.

Producer’s risk

The chance that a lot containing an acceptable quality level will be rejected.