Biostatistics 270 Kruskal-Wallis Test 1. Kruskal-Wallis Test

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1 Biostatistics 270 Kruskal-Wallis Test 1 ORIGIN 1 Kruskal-Wallis Test The Kruskal-Wallis is a non-parametric analog to the One-Way ANOVA F-Test of means. It is useful when the k samples appear not to come from underlying Normal Distributions, or when variance in the different samples are of greatly different magnitudes (non-homogeneous). As with other rank-based tests, it does not have as much power as the fully parametric tests, but nevertheless enjoys wide use. Note that when the number of samples k=2, this test is identical to the Mann-Whitney Test. Data Structure: k groups with not necessarily the same numbers of observations and different means. Let index i,j indicate the ith column (treatment class) and jth row (object). Assumptions: - Observations in each class(block) are a random sample. - Observations in each block are independent of observations in other class. - Underlying distribution of observations in each cell are continuous. - Measurement scale is at least ordinal. One-Way ANOVA Treatment Classes: Objects (Replicates) #1 #2 # #k 1 2 n n1 n2 n nk means: X1bar X2bar Xbar Xkbar Zar Example number loc rank RankSum H 0 : = 0 < No population differences H 1 : < Two Sided Test Hypotheses: Criterion for Normal Approximation: IF n i 5 THEN Normal Approximation Applies OTHERWISE use Special Tables e.g. Rosner Table 15 p. 44

2 Biostatistics 270 Kruskal-Wallis Test 2 Normal Approximation: Rank Data and Sum: - Pool the data over all treatment classes - Total sample size N = n i - Assign Data to Ranks. In the case of ties, t observations in a rank are assigned the appropriate average rank. - Compute the Rank Sum (R i ) for each treatment class i. N 15 k R 64 n R 0 n R 26 n 5 Test Statistic: i 1 12 N ( N 1).72 R i 2 ( N 1) n i i < where R i are the Rank sums for each treatment class i IF no ties, THEN: H < no correction factor in this case... OTHERWISE: m 0 correction factor > 1 m j 1 t j t j N N Sampling Distribution of the Test Statistic H: H x < t represent the number of observations that are tied in groups 1 to g < Corrected Test Statistic If Assumptions hold, H 0 is true, and assuming Normal Approximation then H ~ 2 (k-1) Critical Value of the Test: 0.05 < Probability of Type I error must be explicitly set C qchisq 1 k 1 C < Normal approximation < Note: df = (k-1) Decision Rule: C 5.70 < Critical Value from Zar Appendix B-1 IF H > C THEN REJECT H 0 OTHERWISE ACCEPT H 0 H.72 Probability Value: P ( 1 pchisq( Hk 1) ) P < Normal approximation

3 Biostatistics 270 Kruskal-Wallis Test Zar Example Hypotheses: H 0 : = 0 H 1 : 0 < No population differences < Two Sided Test ph pond rank RankSum n Criterion for Normal Approximation: - IF n i 5 THEN Normal Approximation Applies OTHERWISE use Special Tables e.g. Rosner Table 15 p. 44

4 Biostatistics 270 Kruskal-Wallis Test 4 Normal Approximation: Rank Data and Sum: - Pool the data over all treatment classes - Total sample size N = n i - Assign Data to Ranks. In the case of ties, t observations in a rank are assigned the appropriate average rank. - Compute the Rank Sum (R i ) for each treatment class i. k 4 R Test Statistic: i 1 k 12 R i 2 < where R i are the Rank sums ( N 1) N ( N 1) n for each treatment class i i i IF no ties, THEN: H x < no correction factor... OTHERWISE: 2 m 7 < t represent the number of t 4 tied observations in each of the m groups of tied ranks 2 m t j t j correction factor > 1 j N N H H < Corrected Test Statistic in this case Distribution of the Test Statistic H: n N n N 1 7 If H 0 is true and assuming Normal Approximation then H ~ 2 (k-1) Critical Value of the Test: 0.05 < Probability of Type I error must be explicitly set C qchisq 1 k 1 C < Normal approximation < Note: df = (k-1) Decision Rule: IF H > C THEN REJECT H 0 OTHERWISE ACCEPT H 0

5 Biostatistics 270 Kruskal-Wallis Test 5 Probability Value: P ( 1 pchisq( Hk 1) ) P < Normal approximation Prototype in R: #KRUSKAL WALLIS TEST #ZAR EXAMPLE ZAR=read.table("c:/DATA/Biostascs/ZarEX10.10R.txt") ZAR aach(zar) kruskal.test(number,loc) Kruskal Wallis rank sum test data: number and loc Kruskal Wallis chi squared =.72, df = 2, p value = #ZAR EXAMPLE ZAR=read.table("c:/DATA/Biostascs/ZarEX10.11R.txt") ZAR aach(zar) kruskal.test(ph,pond) Kruskal Wallis rank sum test data: ph and pond Kruskal Wallis chi squared = , df =, p value =