The following glossary contains brief definitions of terms frequently used in reliability engineering and life data analysis. The purpose of these entries is to provide a quick explanation of the terms in question, not to provide extensive explanations or mathematical derivations. For those desiring such detailed descriptions, links have been provided when possible for more extensive coverage elsewhere in ReliaSoft's reliability engineering knowledge base.

For ease of reference, the contents of this Reliability Glossary have also been subdivided into topic-specific categories. View the Subject Listing.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Accelerated life testingfNe中国可靠性资源网
A testing strategy whereby units are tested at stress levels higher than use stress in order to facilitate failures in a timely manner. The results of these tests are then analyzed in such a manner so that a profile of the failure behavior of the products at the use stresses can be determined based on the behavior of the products at the accelerated stresses.fNe中国可靠性资源网
加速试验fNe中国可靠性资源网
为缩短观测产品应力响应所需要持续时间或放大给定持续时间内的响应，在不改变基本的故障模式和失效机理或它们的相对主次关系的前提下，施加的应力水平选取超过规定的基准条件的一种试验。fNe中国可靠性资源网

AMPMfNe中国可靠性资源网
AMPM stands for AMSAA maturity prediction model. This is an enhanced reliability growth model that allows the user to predict failure rates in future stages of development. This model allows the user to assess the effectiveness of proposed and implemented fixes in order to determine the future failure rate.

AMSAA modelfNe中国可靠性资源网
AMSAA stands for Army Material Systems Analysis Activity. This is a reliability growth model that uses a relationship between cumulative test time and cumulative failures to develop a reliability growth model.

ANOVAfNe中国可靠性资源网
ANOVA stands for analysis of variance, a method by which the source of variability is identified. This method is widely used in industry to help identify the source of potential problems in the production process, and identify whether variation in measured output values is due to variability between various manufacturing processes, or within them. By varying the factors in a predetermined pattern and analyzing the output, one can use statistical techniques to make an accurate assessment as to the cause of variation in a manufacturing process.fNe中国可靠性资源网

Arrhenius modelfNe中国可靠性资源网
A model used in accelerated life testing to establish a relationship between absolute temperature and reliability. It was originally developed by Swedish chemist Svante Arrhenius to define the relationship between temperature and the rates of chemical reaction.fNe中国可靠性资源网

AvailabilityfNe中国可靠性资源网
Availability is the probability that an item will be able to function (i.e. not failed or undergoing repair) when called upon to do so. This measure takes into account an item’s reliability (how quickly it fails) and its maintainability (how quickly it can be repaired).fNe中国可靠性资源网
可用性fNe中国可靠性资源网
在要求的外部资源得到保证的前提下，产品在规定的条件下和规定的时刻或时间区间内处于可执行规定功能状态的能力。它是产品可靠性、维修性和维修保障想的综合反映。

BX lifefNe中国可靠性资源网
The time at which X% of the units in a population will have failed. For example, if an item has a B10 life of 100 hours, that means that 10% of the population will have failed by 100 hours of operation.

Block diagramfNe中国可靠性资源网
A diagram that represents how the components, represented by "blocks," are arranged and related reliability-wise in a larger system. This is often but not necessarily the same as the way that the components are physically related. This is also called a Reliability Block Diagram or RBD.

Censored datafNe中国可靠性资源网
Data in which not all of the data points represent failures, i.e. there may be operation times for units that have not failed. Censoring schemes include right-censoring, left-censoring and interval censoring.fNe中国可靠性资源网

Competing failure modesfNe中国可靠性资源网
A model whereby items that fail due to more than one failure mode can be represented as a series reliability system with each block representing a failure mode. The failure modes are considered to be "competing" amongst each other to see which one will cause the item to fail.

Complete datafNe中国可靠性资源网
A data set consisting only of failure times.fNe中国可靠性资源网

Complex systemfNe中国可靠性资源网
A block diagram that cannot be reduced to series and/or parallel systems.fNe中国可靠性资源网
fNe中国可靠性资源网
Confidence boundsfNe中国可靠性资源网
A measure of the precision of a statistical estimate. This is represented by a range of values that the particular estimate should fall between a specified percentage of the time. For example, if we perform ten different reliability tests for our items and analyze the results, we will obtain slightly different parameters for the distribution each time, and thus slightly different reliability results. However, by employing confidence bounds, we obtain a range within which these reliability values are likely to occur a certain percentage of the time. This helps us gauge the utility of the data and the accuracy of the resulting estimates.fNe中国可靠性资源网

Contour plotfNe中国可靠性资源网
A graphical representation of the possible solutions to the likelihood ratio equation. This is employed to determine confidence bounds as well as comparisons between two different data sets.fNe中国可靠性资源网

Cumulative damage modelfNe中国可靠性资源网
An accelerated life testing model used to model accelerated tests where the stress levels vary with time.

Cumulative density function (cdf)fNe中国可靠性资源网
A function obtained by integrating the failure distribution pdf. In life data analysis, the cdf is equivalent to the unreliability function.fNe中国可靠性资源网

Decomposition methodfNe中国可靠性资源网
A method for determining the reliability of complex systems. The decomposition method is an application of the law of total probability, which involves choosing a "key" component and then calculating the reliability of the system twice: once as if the key component failed and once as if the key component succeeded. These two probabilities are then combined to obtain the reliability of the system, since at any given time the key component will be failed or operating.

Degradation analysis退化分析fNe中国可靠性资源网
Analysis involving the measurement and extrapolation of degradation or performance data that can be directly related to the presumed failure of the product in question. Degradation analysis allows the user to extrapolate to an assumed failure time based on the measurements of degradation or performance over time.fNe中国可靠性资源网

Down timefNe中国可靠性资源网
The amount of time a repairable unit is not operating. This can be due to being in a failed state, administrative delay, waiting for replacement parts to be shipped or undergoing active repair.

Duane modelfNe中国可靠性资源网
A reliability growth model similar to the AMSAA model that uses a relationship between cumulative test time and cumulative failures to develop a reliability growth profile.

Event space methodfNe中国可靠性资源网
A method for determining the reliability of complex systems. With the event space method, all mutually exclusive events are determined. The reliability of the system is simply the probability of the union of all mutually exclusive events that yield a system success (the unreliability is the probability of the union of all mutually exclusive events that yield a system failure).

Exponential distributionfNe中国可靠性资源网
A lifetime statistical distribution that assumes a constant failure rate for the units being modeled.fNe中国可靠性资源网

Eyring modelfNe中国可靠性资源网
An accelerated life testing model based on quantum mechanics for use when temperature is the accelerating factor.fNe中国可靠性资源网

Failure distributionfNe中国可靠性资源网
A mathematical model that describes the probability of failures occurring over time. Also known as the probability density function (pdf), this function is integrated to obtain the probability that the failure time takes a value in a given time interval. This function is the basis for other important reliability functions, including the reliability function, the failure rate function and the mean life.fNe中国可靠性资源网

Failure ratefNe中国可靠性资源网
A function that describes the number of failures that can be expected to take place over a given unit of time. The failure rate function has the units of failures per unit time among surviving units, i.e. one failure per month.fNe中国可靠性资源网

Fisher matrixfNe中国可靠性资源网
A mathematical expression that is used to determine the variability of estimated parameter values based on the variability of the data used to make the parameter estimates. It is used to determine confidence bounds when using maximum likelihood estimation (MLE) techniques.fNe中国可靠性资源网

Gaussian distribution fNe中国可靠性资源网
see Normal distribution

General log-linear model fNe中国可靠性资源网
An accelerated life testing model that can account for multiple non-thermal stresses as acceleration factors.

Generalized gamma distributionfNe中国可靠性资源网
While not as frequently used for modeling life data as other life distributions, the generalized gamma distribution does have the ability to mimic the attributes of other distributions such as the Weibull or lognormal, based on the values of the distribution's parameters. While the generalized gamma distribution is not often used to model life data by itself, its ability to behave like other more commonly-used life distributions is sometimes used to determine which of those life distributions should be used to model a particular set of data.fNe中国可靠性资源网

Gompertz modelfNe中国可靠性资源网
A reliability growth model that models reliability values at different stages of development and produces an S-shaped reliability growth curve.

HALTfNe中国可靠性资源网
Highly accelerated life testing.

HASS fNe中国可靠性资源网
Highly accelerated stress screening.

Hazard ratefNe中国可靠性资源网
see Failure rate fNe中国可靠性资源网

Importance measurefNe中国可靠性资源网
A measure of the relative contribution of a component’s contribution to the overall system’s reliability. The importance measure of a component is equivalent to the first partial derivative of the component reliability with respect to the system reliability.

Inverse power lawfNe中国可靠性资源网
An accelerated life testing model commonly used when the accelerating factor is a single, non-thermal stress.fNe中国可靠性资源网

Kaplan-Meier estimatorfNe中国可靠性资源网
This is an estimator used as an alternative to the median ranks method for calculating the estimates of the unreliability for probability plotting purposes. It is also used to determine reliability estimates for nonparametric data analysis.fNe中国可靠性资源网

Life data analysisfNe中国可靠性资源网
The statistical analysis of failure and usage data performed in order to be able to mathematically model the reliability and failure characteristics of a product.

Life distributionfNe中国可靠性资源网
see Failure distribution

Likelihood functionfNe中国可靠性资源网
A function that represents the joint probability of all the points in a data set. For complete data, the likelihood function consists of the product of the pdf for each data point; for data sets that also include suspended or censored data, the likelihood function is more complex. Maximum likelihood estimation (MLE) techniques maximize this function in order to determine the best parameter estimates.fNe中国可靠性资源网

Likelihood ratiofNe中国可靠性资源网
The ratio of a likelihood function for an unknown parameter vector to the likelihood function calculated at the estimated parameter vector. The relationship of this ratio to the chi-squared distribution can then be used to calculate confidence bounds and confidence regions.fNe中国可靠性资源网

Lloyd-Lipow modelfNe中国可靠性资源网
A reliability growth model based on the number of trials and successes at each stage of product development.

Lognormal distributionfNe中国可靠性资源网
A lifetime statistical distribution that is often used to model products in which physical fatigue is the prominent contributor to the primary failure mode.fNe中国可靠性资源网

MaintainabilityfNe中国可靠性资源网
The probability that a failed unit will be repaired within a given amount of time. The term is also used to denote the discipline of studying and improving the maintainability of products, primarily by reducing the amount of time required to diagnose and repair failures.

Maximum likelihood estimation (MLE)fNe中国可靠性资源网
A method of parameter estimation involving the maximization of the likelihood equation. The best parameter estimates are obtained by determining the parameter values that maximize the value of the likelihood equation for a particular data set.fNe中国可靠性资源网

Mean LifefNe中国可靠性资源网
A reliability measure that represents the expected value of the failure times for a failure distribution, also known as the average or central life value. While this represents a useful representative value of a distribution of failure times, it is often over-used as the sole reliability metric.fNe中国可靠性资源网

Median ranksfNe中国可靠性资源网
Measures used to obtain estimates of the unreliability. Median ranks are the values that the true probability of failure should have at the jth failure out of a sample of N units, at a 50% confidence level, or the best estimate for the unreliability. This estimate is based on a solution of the binomial equation.fNe中国可靠性资源网

Mixed Weibull distributionfNe中国可靠性资源网
A variation of the Weibull distribution used to model data with distinct subpopulations that may represent different failure characteristics over the lifetime of a product. Each subpopulation has separate Weibull parameters calculated and the results are combined in a mixed Weibull distribution to represent all of the subpopulations in one function.fNe中国可靠性资源网

Modified Gompertz modelfNe中国可靠性资源网
A reliability growth model that models based on a variation of the Gompertz model.

Monte Carlo simulationfNe中国可靠性资源网
A method of generating values from a known distribution for the purposes of experimentation. This is accomplished by generating uniform random variables and using them in an inverse reliability equation to produce failure times that would conform to the desired input distribution.

MTBFfNe中国可靠性资源网
MTBF stands for mean time before failure and is represented by the mean life value for a failure distribution of non-repairable units.

MTTFfNe中国可靠性资源网
MTTF stands for mean time to failure and is represented by the mean life value for a failure distribution of non-repairable units.

MTTRfNe中国可靠性资源网
MTTR stands for mean time to repair and is represented by the mean life value for a distribution of repair times. (see Maintainability.)

NHPPfNe中国可靠性资源网
NHPP stands for non-homogeneous Poisson process, which is a simple parametric model used to represent events with a non-constant failure recurrence rate. This type of model is often used to model reliability growth and the reliability of repairable units.

Nonparametric analysisfNe中国可靠性资源网
A method of analysis that allows the user to characterize failure data without assuming an underlying failure distribution. This avoids the potentially large errors brought about by making incorrect assumptions about the distribution. However, the confidence bounds associated with nonparametric analysis are usually much wider than those calculated via parametric analysis. Additionally, predictions outside the range of the observations are not possible.fNe中国可靠性资源网

Normal distributionfNe中国可靠性资源网
A common lifetime statistical distribution that was developed by mathematician C. F. Gauss. The distribution is a continuous, bell-shaped distribution which is symmetric about its mean and can take on values from negative infinity to positive infinity.fNe中国可靠性资源网

Path-tracing methodfNe中国可靠性资源网
A method for determining the reliability of complex systems. With this method, every path from a starting point to an ending point is considered. Since system success involves having at least one path available from one end of the reliability block diagram to the other, as long as at least one path is available, the system has not failed. The reliability of the system is simply the probability of the union of these paths.

Plotting paperfNe中国可靠性资源网
see Probability plotting paper

ProbabilityfNe中国可靠性资源网
A quantitative description of the possible likelihood of a particular event. Probability is conventionally expressed on a scale from 0 to 1, or 0% to 100%, with an unlikely event having a probability close to 0, and a very common event having a probability close to 1.

Probability density function (pdf)fNe中国可靠性资源网
A mathematical model that describes the probability of events occurring over time. This function is integrated to obtain the probability that the event time takes a value in a given time interval. In life data analysis, the event in question is a failure, and the pdf is the basis for other important reliability functions, including the reliability function, the failure rate function and the mean life. fNe中国可靠性资源网

Probability plotfNe中国可靠性资源网
A type of plot that linearizes a distribution’s cdf, allowing the user to manually plot failure time vs. estimated unreliability. Provided that the plotted points fall on a relatively straight line (thus indicating that the chosen distribution is a good fit), the parameter estimates can be obtained from scales on the plot. This is a crude, time-consuming method of fitting a distribution to failure data, but it was practically the only method available prior to the widespread use of computers.fNe中国可靠性资源网

Probability plotting paperfNe中国可靠性资源网
A specially designed type of graph paper that allows the user to plot failure time vs. unreliability as a linear function. Plotting paper constructions varies from distribution to distribution. Probability plotting papers that have been generated by ReliaSoft's software are available on the Web at http://www.weibull.com/GPaper/index.htm.fNe中国可靠性资源网

Proportional hazards modelfNe中国可靠性资源网
An accelerated life testing model that can account for multiple non-thermal stresses as acceleration factors.

QualityfNe中国可靠性资源网
A common buzzword referring to the non-quantifiable point-level excellence of a product or process. While sometimes used interchangeably with the term reliability, quality refers to the characteristics of a product at one point in time, while reliability refers to the characteristics of a product over its entire lifetime.

RBDfNe中国可靠性资源网
see Block diagram

ReliabilityfNe中国可靠性资源网
The probability of an item operating for a given amount of time without failure. More generally, reliability is the capability of parts, components, equipment, products and systems to perform their required functions for desired periods of time without failure, in specified environments and with a desired confidence.fNe中国可靠性资源网

Reliability analysisfNe中国可靠性资源网
see Life data analysis

Reliability block diagramfNe中国可靠性资源网
see Block diagram

Reliability growthfNe中国可靠性资源网
The analysis of the change in reliability over time, usually applied to products under development. Reliability growth analysis provides the means by which the reliability, mean life or failure rate is tracked over time, allowing the user to predict future reliability values based on the current rate of growth of the reliability measurement of interest.

Reliability importancefNe中国可靠性资源网
see Importance measure

Reliability life data analysisfNe中国可靠性资源网
see Life data analysis

Reliability test designfNe中国可靠性资源网
The process of designing plans for reliability testing.fNe中国可靠性资源网

Reliability testingfNe中国可靠性资源网
Testing units to failure in order to obtain raw failure time data for life data analysis.

RepairfNe中国可靠性资源网
An action that restores a failed part or component to operating condition.

Repair distributionfNe中国可靠性资源网
A mathematical model that describes the probability of repairs occurring over time.

Repairable systemfNe中国可靠性资源网
A system that can be restored to operating condition after a failure by the repair or replacement of one or more components.

Sequential testingfNe中国可靠性资源网
A testing methodology in which test units are tested consecutively instead of simultaneously.

Spares provisioningfNe中国可靠性资源网
The stocking of spare units or components based on the anticipated number of failures for a given mission or length of operation.

SPRTfNe中国可靠性资源网
SPRT stands for sequential probability ratio test. This is a type of accept/reject sequential testing in which accept/reject boundaries are defined by the user and units are sequentially tested until either the accept boundary or the reject boundary have been reached, and a decision is made about the suitability of the units.

StatisticsfNe中国可靠性资源网
The branch of mathematics that deals with the collection, organization, analysis and interpretation of data.

Stress testingfNe中国可靠性资源网
Testing units at stresses higher than what would be encountered during normal operating conditions, usually to induce failures.

Stress-strength interferencefNe中国可靠性资源网
A method by which the probability of failure of an item is calculated by superimposing the distribution of the item’s strength with the distribution of the stress it will encounter during normal usage.fNe中国可靠性资源网

Suspended datafNe中国可靠性资源网
see Censored data

System reliabilityfNe中国可靠性资源网
The reliability of an entire system, as opposed to the reliability of its components. The system reliability is defined by the reliability of the components as well as the way the components are arranged reliability-wise.

Temperature-humidity modelfNe中国可靠性资源网
An accelerated life testing model used when the two accelerating factors are temperature and humidity.fNe中国可靠性资源网

Temperature-non-thermal modelfNe中国可靠性资源网
An accelerated life testing model used when the two accelerating factors are temperature and another non-thermal stress factor.fNe中国可靠性资源网

Up timefNe中国可靠性资源网
The amount of time a repairable unit is operating per design.

Warranty analysisfNe中国可靠性资源网
The analysis of warranty and return data for the purpose of determining the reliability characteristics of a product.fNe中国可靠性资源网

Weibull distributionfNe中国可靠性资源网
A statistical distribution frequently used in life data analysis. Developed by Swedish mathematician Wallodi Weibull, this distribution is widely used due to its versatility and the fact that the Weibull pdf can assume different shapes based on the parameter values.fNe中国可靠性资源网