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D
DECISION RULE
A rule for comparing the OUTCOME VALUE of ALPHA with a NOMINAL ALPHA CRITERION LEVEL (such as 0.05). An OUTCOME VALUE smaller (more extreme) than the NOMINAL ALPHA CRITERION LEVEL leads to a decision of STATISTICAL SIGNIFICANCE of the finding that the TEST STATISTIC has a value other than its (null-) hypothesised value. Also see : STATISTICAL SIGNIFICANCE, TAIL-DEFINITION POLICY.
DEGREES OF FREEDOM
An integer value measuring the extent to which an EXPERIMENTAL DESIGN imposes constraints upon the pattern of the mean values of data from various meaningful subsets of data. This value is frequently referred to in the organisation of tables of statistical distributions used in undertaking SIGNIFICANCE TESTS. For simple one-way classifications the value of DEGREES OF FREEDOM is defined as one less than the number of subsets.
DIFFERENCE OF MEANS
A TEST STATISTIC of intuitive appeal for measuring difference in location between two samples with INTERVAL-SCALE data. Employing this TEST STATISTIC in an EXACT TEST defines the PITMAN PERMUTATION TESTs(1 or 2).
DISCRETE DISTRIBUTION
A probability distribution of some STATISTIC, based upon an algebraic formula or upon re-randomisation or upon actual data, in which the cumulative probability increases in non-infinitesmal steps corresponding to non-infinitesmal weight associated with possible values of the STATISTIC in question. This situation is characteristic of RANDOMISATION DISTRIBUTIONs, and also of TEST STATISTICs which are essentially discrete. Also see : CONTINUOUS DISTRIBUTION.
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D