SPC Glossary 

Your quick reference to statistical process control for manufacturing quality managment systems.

Quality Management System Glossary:
H, I, J, K



A diagram consisting of rectangles whose area is proportional to the frequency of a variable and whose width is equal to the class interval. Gives a rough sense of the density of the underlying distribution of the data and is often used for density estimation—that is, estimating the probability density function of the underlying variable. The total area of a histogram used for probability density is always normalized to 1.

Hypothesis Testing

A procedure that is used on a sample from a population to investigate the applicability of an assertion (inference) to the entire population. Hypothesis testing can also be used to test assertions about multiple populations using multiple samples.



A quality characteristic’s departure from its intended level or state without any association to conformance to specification, requirements, or to the usability of a product or service. Also see Blemish, Defect, and Nonconformity.

In-Control Process

A process in which the statistical measure being evaluated is in a state of statistical control; in other words, the variations among the observed sampling results can be attributed to a constant system of chance causes. Also see Out-of-Control Process.


A single unit or a single measurement of a quality characteristic, usually denoted as X. This measurement is analyzed using an Individuals Chart, CUSUM, or EWMA chart.

Individuals Chart

Also called an I-chart or X-chart.

A control chart for processes in which individual measurements of the process are plotted for analysis. 


A verification activity. 

For example, measuring, examining, testing, and gauging one or more characteristics of a product or service and comparing the results with specified requirements to determine whether conformity is achieved for each characteristic.

Inspection Cost

The cost associated with inspecting a product to ensure it meets the internal or external customer’s needs and requirements; an appraisal cost.

Inspection Lot

This is the lot or batch of product to be inspected for acceptance.

Inspection States 

In the ANSI/ASQ and ISO Acceptance Sampling Standards there are three Inspection States (or statuses): Normal, Tightened, and Reduced. The definitions for each state are found in the applicable standard under a heading called Switching Rules.
  1. Normal: Per the ANSI/ASQ z1.4, “Normal inspection will be used at the start of inspection unless otherwise directed by the responsible authority.”
  2. Normal to Tightened: Per the ANSI/ASQ z1.4, “When Normal inspection is in effect, tightened inspection shall be instituted when 2 out of 5 or fewer consecutive lots or batches have been non-acceptable on original inspection.”
  3. Tightened to Normal: Per the ANSI/ASQ z1.4, “When tightened inspection is in effect, normal inspection shall be instituted when 5 consecutive lots or batches have been considered acceptable on original inspection.”
  4. Normal to Reduced: This switching rule requires several different things to happen. When normal inspection is in effect and the following apply, the state may change to Reduced if:
  • The preceding 10 lots or batches (or more) have all been accepted on original inspection
  • The total number of nonconforming units in the samples from those 10 preceding lots is equal to or less than the applicable limit number given (depending on the standard)
  • Production is at a steady rate
  • Reduced inspection is desired by responsible authority

International Organization for Standardization (ISO)

An independent, nongovernmental international organization with a membership of 161 national standards bodies that unites experts to share knowledge and develop voluntary, consensus-based, market-relevant international standards, guidelines, and other types of documents.

ISO 9000 Series Standards

A set of international standards on quality management and quality assurance developed to help organizations effectively document the quality system elements to be implemented to maintain an efficient quality system. The standards, initially published in 1987, are not specific to any particular industry, product, or service. The standards were developed by the International Organization for Standardization (ISO). The standards underwent major revision in 2000 and now include ISO 9000:2005 (definitions), ISO 9001:2008 (requirements), ISO 9004:2009 (continuous improvement) and ISO 9001: 2015 (risk management).

ISO 9001

A voluntary quality management system standard developed by the International Organization for Standardization (ISO). First released in 1987 and one of several documents in the ISO 9000 family.


Just-In-Time Manufacturing (JIT)

Also known as Just-In-Time Production.

A methodology aimed primarily at reducing flow times within a production system, as well as response times from suppliers and to customers. 


Key Process Characteristic 

A process parameter that can affect safety or compliance with regulations, fit, function, performance or subsequent processing of product.

Key Product Characteristic 

A product characteristic that can affect safety or compliance with regulations, fit, function, performance or subsequent processing of product.

Kruskal-Wallis Test

A non-parametric test for determining whether samples originate from the same distribution. It is used for comparing two or more independent samples of equal or different sample sizes. While analysis of variance tests depends on the assumption that all populations under comparison are normally distributed, the Kruskal-Wallis test places no such restriction on the comparison. It is a logical extension of the Wilcoxon Mann-Whitney Test.

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