Z94.16 - Quality Assurance & Reliability

Most definitions were taken from existing Terminology standards with appropriate references.

Russell G. Heikes
Georgia Institute of Technology

Harrison Wadsworth, Jr.
Professor Emeritus
Georgia Institute of Technology


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ACCELERATED TEST.  Test in which the applied stress level is chosen to exceed that stated in the reference conditions in order to shorten the time required to observe the stress response of the item, or to magnify the response in a given duration.  To be valid, an accelerated test must not alter the basic modes and/or mechanisms of failure. [4: 191-14-0]

ACCEPTABLE PROCESS LEVEL (APL).  Process level which forms the outer boundary of the zone of process levels which are acceptable. [2: 3.4.10]

ACCEPTABLE QUALITY LEVEL (AQL).  When a continuing series of lots is considered, a quality level which for the purposes of sampling inspection is the limit of a satisfactory process average.  Note: The value of the AQL selected is usually dependent upon physical and economic constraints such as the natural process limits (which determine the tolerances that can be set for various technical characteristics) and the costs of inspection balanced against the costs of failure in service. [2: 2.7.1]

ACCEPTANCE CONTROL CHART.  A graphical method for the dual purposes of evaluating a process in terms of: (a)whether or not it can be expected to satisfy product or service requirements for the characteristic(s) being measured, and (b)whether or not it is in a "state of statistical control" with respect to within-sample or sub-group variability.  Notes: (1) For variables data, this will require one chart for averages and another for ranges or standard deviations. (2) The emphasis of the acceptance control chart is that the process usually does not need to remain in control about some single standard process level, but as long as the within-sub-group variability remains in control it can run at any level within a zone of process levels acceptable in terms of the process requirements.  Some assignable causes will create shifts in the process level which are small when compared with requirements and it is uneconomical to control them too tightly. The issue of narrowing the zone around the target usually involves a different set of problems and actions from problems of process instability within sub-groups. [2: 3.3.16]

ACCEPTANCE CRITERIA.  Specification criteria for acceptance of individual product or service characteristics.  Note: Sometimes, as in acceptance sampling, the term "acceptance criteria" is used for a set of several characteristics rather than an individual characteristic.  [2: 1.1.10]

ACCEPTANCE INSPECTION.  Inspection to determine whether an item or lot delivered or offered for delivery is acceptable. [2: 1.2.3]

ACCEPTANCE NUMBER, (AC).  In sampling inspection by attributes, the largest number of nonconformities or nonconforming items found in the sample that permits the acceptance of the lot, as given in the sampling plan. [2: 2.3.10]

ACCEPTANCE SAMPLING. Sampling inspection in which decisions are made to accept or not to accept a lot (or other grouping of product, material, or service) based on the results of a sample or samples selected from that lot. (See SAMPLING INSPECTION.)  Notes: (1) The alternative to acceptance is often termed "rejection" for the purpose of the definition.  However,  in practice, the alternative may take some form other than outright rejection. (2) In lot-by-lot sampling, acceptance and rejection relate to individual lots.  In continuous sampling, acceptance and rejection relate to individual units or to blocks of consecutive units, depending on the stated procedure. [2: 2.3.1]

ACCURACY.  The closeness of agreement between a test result and the accepted reference value.  Note: The term accuracy, when applied to a set of test results, involves a combination of random components and a common systematic error or bias component. [1: 3.11]

ACTION LIMITS. ACTION CONTROL LIMITS (UPPER AND/OR LOWER):  In a control chart, the limits above which (upper limit) or below which (lower limit) or the limits outside which the statistic under consideration lies when action should be taken. [2: 3.4.3]

ARITHMETIC MEAN; AVERAGE.  The sum of values divided by the number of values.  Notes: (1) The term "mean" is used generally when referring to a population parameter and the term "average" when referring to the result of a calculation on the data obtained in a sample. (2)The average of a simple random sample taken from a population is an unbiased estimator of the mean of this population.  However, other estimators such as the geometric or harmonic mean, or the median or mode are sometimes used.[1: 2.26]

ARITHMETIC WEIGHTED MEAN. Weighted Average:  The sum of the products of each value and its weight, divided by the sum of the weights where weights are non-negative coefficients assigned to each of the values. [1: 2.27]

ASSIGNABLE CAUSE.  A factor (usually systematic) that can be detected and identified as contributing to a change in a quality characteristic or process level.  Notes: (1) Assignable causes are sometimes referred to as special causes of variation. (2) Many small causes of change are assignable, but it may be uneconomic to consider or control them.  In that case they should be treated as chance causes. [2: 3.1.8]

AVAILABILITY (ACHIEVED).  The probability that an item when used and maintained under stated conditions in an ideal support environment will be in a satisfactory state at any given time.  It may be expressed as: Aa(t) = MTBM /  MTBM+M, where  MTBM = mean time between maintenance - is a function of both preventive and corrective maintenance requirements.  M = mean active maintenance downtime - a value which is a function of the repair times associated with corrective and preventive maintenance.

AVAILABILITY (INTRINSIC).  The probability that an item is in satisfactory state at a stated instant of time (t) when it is used and maintained under stated conditions.  It may be expressed as:  Ai(t) = MTBF / MTBF +MTTR.  MTBF = mean time between failures. MTTR = mean time to repair (excludes preventive maintenance downtime, supply down-time, and administrative down time).

AVAILABILITY (OPERATION).  The probability that an item is in a satisfactory state at a stated instant of time(t) when it is used and maintained under stated conditions in actual support environment.  It may be expressed as:  Ao(t) = MTBM  / MTBM + MDT.  where,  MTBM = mean time between maintenance.  MDT = mean downtime (includes preventive maintenance, supply and administrative downtime).

AVAILABILITY (PERFORMANCE).  The ability of an item to be in a state to perform a required function under given conditions at a stated instant of time or over a given time interval. [4: 191-02-05]

AVERAGE AMOUNT OF INSPECTION.  In a given sampling scheme, the number of items expected to be inspected per lot in order to reach a decision for a certain average batch quality level.  Note: This is an average over switching rules, etc., unlike average sample number.  It does not include inspecting all items in non-accepted lots as average total inspected requires.  (See AVERAGE SAMPLE NUMBER, AVERAGE TOTAL INSPECTED.) [2: 2.7.7]

AVERAGE CHART;  X OR Y CHART.  A control chart for evaluation the sub-group differences in terms of the sub-group averages. [2: 3.3.4]

AVERAGE OUTGOING QUALITY (AOQ).  The expected average quality level of outgoing product for a given value of incoming product quality.  Note:  In practical cases different definitions of AOQ may be used depending on whether or not nonconforming items removed in the 100% inspection of non-accepted lots are replaced by conforming items. [2: 2.7.4]

AVERAGE OUTGOING QUALITY LIMIT (AOQL).  Maximum AOQ over all possible values of incoming product quality level for a given acceptance sampling plan and rectification of all non-accepted lots. [2: 2.7.5]

AVERAGE RUN LENGTH (ARL). (1) sample sense: The average number of times that a process will have been sampled and evaluated before a shift in process level is signalled. (2) item sense: The average number of items that will have been produced before a shift in level is signalled.  Note: A long ARL is desirable for a process located at its specified level (so as to minimize calling for unneeded investigation or corrective action); a short ARL is desirable for a process shifted to some undersirable level (so that corrective action will be called for promptly).  ARL  curves are used to describe the relative quickness in detecting level shifts of various control chart systems. [2: 3.3.15]

AVERAGE RANGE; MEAN RANGE.  The arithmetic mean of the ranges of a set of samples of the same size. [1: 2.31]

AVERAGE SAMPLE NUMBER (ASN).   Average sample size:  Average number of sample units inspected per lot in reaching decisions to accept or not to accept when using a given sampling plan.  Note: ASN is dependent on the actual quality level of the submitted lots.  (See AVERAGE AMOUNT OF INSPECTION.) [2: 2.7.6]

AVERAGE TOTAL INSPECTED (ATI).  Average number of items inspected per lot including inspection of all items in non-accepted lots.  Note: Applicable when the procedure calls for 100% inspection of non-accepted lots.  [2: 2.7.8]


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