Next: Testing strategies for monotonic Up: Qualitative Trends And Trend Previous: Monotonic trend as an
Some testing strategies aiming at monotonic trendsA variety of testing strategies come in question when considering statistical hypotheses about monotonic trends. The application of a global F test of analysis of variance, followed by a differential and databased interpretation seems to be the most widespread. As has been argued above, this procedure is problematic, however, as the differential interpretation is not in accordance with the acceptance of the of the global F test and besides, an uncontrollable inflation of statistical error probabilities will occur. Thus the probability of a wrong decision in favor of strict monotonicity can be significantly increased, exceeding the prechosen by a substantial amount. Furthermore, this manner of testing a particular qualitative trend hypothesis does not fulfill the criteria of appropriateness (directional differences have been predicted, but the test refers to nondirectional differences) and exhaustiveness (directional differences between all or at least some means, but the F test can also come out significant, if a large difference is associated with an inversion of rank order). Another procedure, which is sometimes recommended, is to perform an F test for the hypothesis about the quantitative trend that is formulated to match the qualitative trend of interest: In place of the hypothesis of a strictly monotonic trend the test refers to the respective linear component through the orthogonal polynomials (see, e.g., Levin & Marascuilo, 1972, pp. 372373 [40]). However, as has been stated above, the test can turn out statistically significant even if one or more rank inversions occur and since it may remain insignificant even if the rank order of means meets the prediction, but the differences among the means are inhomogeneous. The latter case is in accordance to the (qualitative) trend hypothesis of interest, the former is not. Thus, the interpretation of the results is ambiguous with respect to the hypothesis of strict monotonicity. But what would the consequences be if Testing Strategy TS 1, outlined above, is applied? Applying Testing Strategy TS 1, the SHmon is decomposed into the testable conjunction of hypotheses '' from Expression (2). If both hypotheses are accepted the trend is strictly monotonic by implication, although it is not possible to infer that all distances are large enough to reach statistical significance if tested separately as has been chosen as an additional criterion above. If, on the other hand, the two tests lead to accepting one of the conjunctions of hypotheses '' or '', respectively, it can be concluded that there is no monotonic trend. Yet if the pattern of decisions is '' the 'presence' or 'absence' of a strictly monotonic trend cannot be inferred unambiguously and testbased, since deviations from linearity () can be caused either by unequal distances among increasing or decreasing ranks or by rank inversions across the J means. Unequal distances again are compatible with strict monotonicity, whereas inverted ranks are not. Additionally, it remains unclear again whether the demand is fulfilled that adjacent means differ significanctly for each pair of means. Overall, the interpretation of the outcomes of the respective tests are ambiguous with respect to strict monotonicity. Thus, Smith and Macdonald (1983, p. 3) [61] conclude with respect to the procedure just outlined that these tests may be 'optimal,' 'when the true state of the world is a linear trend. When the intervals between successive ... () are not equal or are not known (and this is very commonly the case in psychology) the linear trend procedure is suspect and alternatives need to be examined.'
The method of orthogonal contrasts, whether to be
used following a significant F test or instead of it, is
covered in all textbooks and is in frequent use. Therefore, the
question arises whether a 'satisfactory' testing strategy
can be devised for hypotheses about orthogonal contrasts, enabling
a testbased decision about a strictly monotonic trend.
Without going into the details it can be stated that a strict
rank order across J means cannot be established without
supplementing testbased propositions to a large degree with databased
ones (see Hager, 1992, pp. 365368 [22],
for the details). For this
reason, further alternatives to the quantitative trend tests described
up to point are in demand. Another procedure consists of applying
modified (quantitative) trend tests. The modification mainly concerns
the choice of a set of 'optimum' contrast coefficients
according to the proposals made by
Abelson and Tukey (1963 [1]; see for an application
Bortz, 1993, pp. 259260 [5]). For comprehensive
and comparative surveys of these and further tests see Berenson
(1982) [3] and
Smith and Macdonald (1983) [61].
According to Barlow, Bartholomew,
Bremner and Brunk (1972, p. 118, p. 194) [2],
Berenson (1982, p. 270) [3],
and Le (1987, p. 173) [38], the hypotheses ( and
) tested against each other by
these and related tests are:
The numerous procedures of ordering and selection (see above) and several multistage procedures combining the parametric F test with a rank correlation (Chassan, 1960 [9]; Green & NimmoSmith, 1982 [20]; Macdonald & Smith, 1983 [42]) seem to test analoguous statistical hypotheses. At least, the statistical hypothesis of a strictly monotonic trend is addressed in neither case, as Macdonald and Smith (1983, p. 25) [42] have pointed out, raising the question of further alternative procedures once again. Only one of presumably various possibilities is described here, in which the testing of hypotheses about qualitative trends is interpreted as a problem of testing hypotheses by means of planned a priori or focussed contrasts, the manner usually advocated when examining hypotheses formulated in advance (see, among others, Kirk, 1982 [36]; Marascuilo & Levin, 1983, p. 337 [43]; Thompson, 1994 [62]), but rarely applied in psychological research practice. As to my own experience, one of the reasons for this may be that many reviewers demand overall tests which may be followed by a multiple comparison procedure. But Winer et al. (1991, p. 146) [68] clearly state: 'A procedure which is appropriate for a series of planned ... [contrasts] is simply to carry out a series of t tests, where t is appropriately defined for the experimental design used' ['comparisons' replaced by 'contrasts']. But this method seems to be suspect to many researchers who rely on overall tests even if particular contrast hypotheses can be formulated in advance. Maybe a series of t tests is too simple a procedure to be 'scientific', even if there is good reason to perform them?
Next: Testing strategies for monotonic Up: Qualitative Trends And Trend Previous: Monotonic trend as an Methods of Psychological Research 1996, Vol.1, No.4 © 1997 Pabst Science Publishers 
