- Van der Waerden test
Named for the Dutch mathematician
Bartel Leendert van der Waerden , the Van der Waerden test is astatistical test that "k" population distribution functions are equal. The Van Der Waerden test converts the ranks from a standardKruskal-Wallis one-way analysis of variance toquantile s of the standard normal distribution (details given below). These are called normal scores and the test is computed from these normal scores.Background
Analysis of Variance (ANOVA) is adata analysis technique for examining the significance of the factors (independent variables ) in a multi-factor model. The one factor model can be thought of as a generalization of thetwo sample t-test . That is, the two sample t-test is a test of the hypothesis that two population means are equal. The one factor ANOVA tests the hypothesis that "k" population means are equal. The standard ANOVA assumes that the errors (i.e., residuals) are normally distributed. If this normality assumption is not valid, an alternative is to use anon-parametric test .Test definition
Let "ni" ("i" = 1, 2, ..., "k") represent the sample sizes for each of the "k" groups (i.e., samples) in the data. Let "N" denote the sample size for all groups. Let "Xij" represent the "i"th value in the "j"th group. The normal scores are computed as :with "R"("Xij") denoting the rank of observation "Xij" and φ denoting the normal
quantile function . The average of the normal scores for each sample can then be computed as:The variance of the normal scores can be computed as:The Van Der Waerden test can then be defined as follows::H0: All of the "k" population distribution functions are identical:Ha: At least one of the populations tends to yield larger observations than at least one of the other populationsThe test statistic is:For
significance level α, the critical region is:where Χα,k − 12 is the α-quantile of thechi-square distribution with "k" − 1 degrees of freedom. The null hypothesis is rejected if the test statistic is in the critical region. If the hypothesis of identical distributions is rejected, one can perform amultiple comparisons procedure to determine which pairs of populations tend to differ. The populations "i" and "j" seem to be different if the following inequality is satisfied::with "t"1 − α/2 the (1 − α/2)-quantile of thet distribution .Comparison with the Kruskal-Wallis test
The most common non-parametric test for the one-factor model is the Kruskal-Wallis test. The Kruskal-Wallis test is based on the ranks of the data. The advantage of the Van Der Waerden test is that it provides the high efficiency of the standard ANOVA analysis when the normality assumptions are in fact satisfied, but it also provides the robustness of the Kruskal-Wallis test when the normality assumptions are not satisfied.
References
*cite book
first= W. J.
last=Conover
year = 1999
title = Practical Nonparameteric Statistics
edition = Third Edition
publisher = Wiley
pages = 396-406
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