SPC Glossary: S

Quality Management System Glossary: S


In acceptance sampling, one or more individual units (pieces) of product drawn from a lot for purposes of inspection to reach a decision regarding acceptance of the lot.

Sample Size [n]

The number of units (pieces) in a sample.

Sample Standard Deviation Chart (chart)

The chart tracks subgroup standard deviations; the plot point represents the calculated sample (n-1) standard deviation of the subgroup.

Sampling at Random

As commonly used in acceptance sampling theory, the process of selecting sample units so all units under consideration have the same probability of being selected.

Note: Equal probabilities are not necessary for random sampling; what is necessary is that the probability of selection be ascertainable. However, the stated properties of published sampling tables are based on the assumption of random sampling with equal probabilities. An acceptable method of random selection with equal probabilities is the use of a table of random numbers in a standard manner. A simple random sample is a set of n objects in a population of N objects where all possible samples are equally likely to happen.

Example: 100 objects (n) in a population of 10,000 objects (N). In Acceptance Sampling, the Lot size combined with the AQL defines how many “random” samples to inspect.

Sampling Distribution

The probability distribution of a statistic. Common sampling distributions include t, chi-square (c2), and F. Also known as finite-sample distribution, sampling distribution is the probability distribution of a given random-sample-based statistic. Sampling distributions are important in statistics because they provide a major simplification en route to statistical inference.

Sampling, Single

Sampling inspection in which the decision to accept or reject a lot is based on the inspection of one sample. A single sampling plan is specified by the pair of numbers (n,c). The sample size is n, and the lot is rejected if there are more than c defectives in the sample. It is referred to as single, because the decision is made on one inspection (visual or measured) of 1 or more pieces.

Lot size = 500, AQL is 0.25, sample size (n) = 50, c=1. If any piece is outside specification, the lot (or sample) fails.

Sampling, Unit

Sequential sampling inspection in which, after each unit is inspected, the decision is made to accept a lot, reject it or inspect another unit. See Single Sampling above.

Example from the web:
In the context of market research, a sampling unit is an individual person. The term sampling unit refers to a singular value within a sample database. For example, if you were conducting research using a sample of university students, a single university student would be a sampling unit.

Scatter Plots

A graphical technique used to visually analyze the relationship between two variables. Two sets of data are plotted on a graph: the y-axis indicates the variable to be predicted, and the x-axis indicates the variable to make the prediction.

Short-Run Techniques

Adaptations made to control charts to help determine meaningful control limits when only a limited number of parts are produced, or when a limited number of services are performed. Short-run techniques usually focus on the deviation (of a quality characteristic) from a target value.


One standard deviation in a normally distributed process.

Six Sigma

A rigorous, data-driven approach (and methodology) for analyzing and eliminating the root causes of business problems.

Six Sigma Black Belt (BB)

Also known as Lean Six Sigma Black Belt and Black Belt Six Sigma.

Certified Lean Six Sigma designation. A full-time team leader responsible for implementing process improvement projects—define, measure, analyze, improve and control (DMAIC) or define, measure, analyze, design and verify (DMADV)—within a business to drive up customer satisfaction and productivity levels.

Six Sigma Green Belt (GB)

An employee who has been trained in the Six Sigma improvement method and can lead a process improvement or quality improvement team as part of his/ her full-time job.

Six Sigma Master Black Belt (MBB)

Also known as Lean Six Sigma Master Black Belt.

A problem-solving subject matter expert responsible for strategic implementations in an organization. This Six Sigma pro is typically qualified to teach other facilitators the statistical and problem-solving methods, tools, and applications to use in such implementations.

Six Sigma Tools

The problem-solving tools used to support Six Sigma and other process improvement efforts: voice of the customer, value stream mapping, process mapping, capability analysis, Pareto charts, root cause analysis, failure mode and effects analysis, control plans, statistical process control, 5S, mistake proofing, and design of experiments.

Six Sigma Yellow Belt

Refers to someone who has attained Six Sigma yellow belt certification. A team member who supports and contributes to Six Sigma projects, often helping to collect data, brainstorm ideas, and review process improvements.


Asymmetry in a statistical distribution. Skewed data may affect the validity of control charts and other statistical tests based on the normal distribution.

Special Cause

Cause of variation that arise because of special circumstances. They are not an inherent part of a process. Special cause is also referred to as assignable cause. Also see Common Cause.


A document that states the requirements to which a given product or service must conform.


Also known as dispersionvariability, or scatter.

The extent to which a distribution is stretched or squeezed.


A stable process is said to be in control. A process is considered stable if it is free from the influences of special causes.

Standard Deviation (statistical)

A measure that is used to quantify the amount of variation or dispersion of a set of data values.


A single measure of some attribute of a sample—used to make inferences about the population from which the sample came. Sample mean, median, range, variance, and standard deviation are commonly calculated statistics.

Statistical Process Control (SPC)

An industry-standard methodology for measuring and controlling quality during the manufacturing process.

Statistical Quality Control (SQC)

The application of statistical techniques to control quality. Includes acceptance sampling, which statistical process control does not.


A branch of mathematics dealing with the collection, organization, analysis, interpretation, and presentation of data.


Another name for a sample from the population.

Supplier Quality Assurance

Confidence that a supplier’s product or service will fulfill its customers’ needs; achieved by creating a relationship between the customer and supplier that ensures the product will be fit for use with minimal corrective action and inspection.

According to quality management guru Joseph M. Juran, nine primary activities are needed: 1) define product and program quality requirements; 2) evaluate alternative suppliers; 3) select suppliers; 4) conduct joint quality planning; 5) cooperate with the supplier during the execution of the contract; 6) obtain proof of conformance to requirements; 7) certify qualified suppliers; 8) conduct quality improvement programs as required; and 9) create and use supplier quality ratings.

Supplier Quality Management

A system in which supplier quality is managed by using a proactive and collaborative approach. The costs of transactions, communication, problem resolution, the impact of switching suppliers, and overall cost. Also focuses on factors that impact supply-chain performance, such as the reliability of the supplier delivery, and the supplier’s internal policies regarding inventory levels.

Supply Chain

The system of organizations, people, activities, information, and resources involved in moving a product or service from supplier to customer.