# Glossary

**2 ^{k} factorial experiment.** A full factorial experiment with

*k*factors and all factors tested at only two levels (settings) each.

**2 ^{k–p} factorial experiment.** A fractional factorial experiment with

*k*factors tested in a 2

^{–p}fraction with all factors tested at only two levels (settings) each.

**Acceptance region.** In hypothesis testing, a region in the sample space of the test statistic such that if the test statistic falls within it, the null hypothesis cannot be rejected. This terminology is used because rejection of H_{0} is always a strong conclusion and acceptance of H_{0} is generally a weak conclusion.

**Addition rule.** A formula used to determine the probability of the union of two (or more) events from the probabilities of the events and their intersection(s).

**Additivity property of χ ^{2}.** If two independent random variables

*X*

_{1}and

*X*

_{2}are distributed as chi-square with

*v*

_{1}and

*v*

_{2}degrees of freedom, respectively,

*Y*=

*X*

_{1}+

*X*

_{2}is a chi-square random variable with

*u*=

*v*

_{1}+

*v*

_{2}degrees of freedom. This generalizes to any number of independent chi-square random variables.

**Adjusted R ^{2}.** A variation of the

*R*

^{2}statistic that compensates for the number of parameters in a regression model. Essentially, the adjustment is a penalty for increasing the number of parameters in the model.

**Alias.** In a fractional factorial experiment when certain factor effects cannot be estimated uniquely, they are said to be aliased.

**All possible (subsets) regressions.** A method of variable selection in regression that ...

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