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EARLY FAILURE PERIOD. That possible early period beginning at some stated time and during which the failure rate is decreasing rapidly in comparison with the subsequent period.
ELEMENTARY MATRIX. A unit or identity matrix except for one unique column or row. The inverse of a non singular elementary matrix is also an elementary matrix. 
ELIMINATION METHOD. An algorithm for solving a set of m x m independent linear equations. (See GAUSSIAN ELIMINATION.) 
EQUATION. A proposition (or relation) that a given mathematical function shall be equal to another function (often a constant). 
ERROR MEAN-SQUARE. The residual or error sum-of-squares (q.v.) divided by the number of degrees of freedom on which the sum is based. It is an estimator of the residual or error variance. 
ERROR SUM OF SQUARES. In the analysis of variance and covariance, and regression analysis, it is the sum of squares that remains after subtracting from the total sum of squares all sums of squares associated with the factors of the experiment. It also is referred to as the residual sum of squares.
ESTIMATION. Inference about the numerical value of unknown population values from incomplete data such as a sample. If a single figure is calculated for each unknown parameter the process is called point estimation. If an interval is calculated within which the parameter is likely, in some sense, to lie, the process is called interval estimation. 
EVENT. An occurrence of some attribute. In strict probability terminology, it is a subset of the sample space.
EVOLUTIONARY OPERATION. Method of experimentation during production, whereby process variables are changed in a systematic manner and the results are observed. The changes are small in order to avoid the risk of off-standard product or other undesirable effects. Statistical analysis provides guides toward process improvement in the form of increased yield or improved quality.
EXPONENTIAL SERVICE TIME. The service time is a random variable with an negative exponential distribution.
EXTREMAL PROBLEM. (See MASTER PROGRAM.)
EXTREME POINT. A point of a convex set which does not lie on a line segment joining any other two points of the set; also called a “vertex.” 
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