# Difference tests (2): nonparametric

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1 NST 1B Experimental Psychology Statistics practical 3 Difference tests (): nonparametric Rudolf Cardinal & Mike Aitken 10 / 11 February 005; Department of Experimental Psychology University of Cambridge Slides at pobox.com/~rudolf/psychology

2 These slides are on the web. No need to scribble frantically. pobox.com/~rudolf/psychology

3 Nonparametric tests Last time, we looked at the t test, a parametric test it made assumptions about parameters of the underlying populations (such as the distribution e.g. assuming that the data are normally distributed). If these assumptions are violated: (a) we could transform the data to fit the assumptions better (NOT covered at Part 1B level) or (b) we could use a nonparametric ( distribution-free ) test that doesn t make the same assumptions. In general, if the assumptions of parametric tests are met, they are the most powerful. If not, we may need to use nonparametric tests. They may, for example, answer questions about medians rather than means. We ll look at some nonparametric tests now that assume only that the data are measured on at least an ordinal scale.

4 The median The median is the value at or below which 50% of the scores fall when the data are arranged in numerical order. (Can also be referred to as the 50 th centile.) If n is odd, it s the middle value (here, 17): If n is even, it s the mean of the two middle values (here, 17.5): the two middle values The median is (17+18) = 17.5

5 The median The median is the value at or below which 50% of the scores fall when the data are arranged in numerical order. (Can also be referred to as the 50 th centile.)

6 Medians are less affected by outliers than means

7 Ranking removes distribution information

8 How to rank data Suppose we have ten measurements (e.g. test scores) and want to rank them. First, place them in ascending numerical order: Then start assigning them ranks. When you come to a tie, give each value the mean of the ranks they re tied for for example, the 1s are tied for ranks 4 and 5, so they get the rank 4.5; the 16s are tied for ranks 7, 8, and 9, so they get the rank 8: X: rank:

9 Nonparametric correlation We ve already seen this.

10 r s : Spearman s correlation coefficient for ranked data We met this in the first statistics practical. It s a nonparametric version of correlation. You can use it when you obtain ranked data, or when you want to do significance tests on r but your data are not normally distributed (violating an assumption of the parametric t test that s based on Pearson s r). Rank the X values. Rank the Y values. Correlate the X ranks with the Y ranks. (You do this in the normal way for calculating r, but you call the result r s.) To ask whether the correlation is significant, use the table of critical values of Spearman s r s in the Tables and Formulae booklet.

11 Nonparametric difference tests

12 Two unrelated samples: the Mann Whitney U test Logic Suppose we have two samples with n 1 and n observations in each (for a total of n 1 + n = N observations). We can rank all observations together, from 1 to N. If the two samples come from identical populations, the sum of the ranks of sample 1 scores is likely to be about the same as the sum of the ranks of sample scores. But if sample 1 comes from a population with much lower values than sample, the sum of the ranks of sample 1 scores will generally be lower than the sum of the ranks of sample scores. Null hypothesis: the two samples were drawn from identical populations. [Unlike the unpaired t test, whose null hypothesis was that the two samples came from populations with the same mean.] If we assume the distributions are similar, a significant Mann Whitney test suggests that the medians of the two populations are different.

13 Calculating the Mann Whitney U statistic From the Formula Sheet: Calculating the Mann Whitney U statistic 1. Call the smaller group group 1, and the larger group group, so n 1 < n. (If n 1 = n, ignore this step.). Calculate the sum of the ranks of group 1 (= R 1 ) and group (= R ). 3. n 1 ( n 1 + 1) U1 = R1 4. n ( n + 1) U = R 5. The Mann Whitney statistic U is the smaller of U 1 and U. Check your sums: verify that U 1 + U = n 1 n and ( n1 + n)( n1 + n + 1) R 1 + R =.

14 Mann Whitney U test: EXAMPLE 1 Mothers either received care from first trimester onwards birth weights (in kg): {1.68, 3.83, 3.11,.76, 1.70,.79, 3.05,.66, 1.40,.775} or from the third trimester onwards babies birth weights: {.94, 3.38, 4.90,.81,.80, 3.1, 3.08,.95}. Is there a significant difference between the birthweights of the two groups? Third trimester group smaller, so is group 1 (n 1 = 8). Other is group (n = 10). Ranks for group 1: {10, 16, 18, 9, 8, 15, 13, 11}; rank sum R 1 = 100. Ranks for group : {, 17, 14, 5, 3, 7, 1, 4, 1, 6}; rank sum R = 71. n 1 ( n 1 + 1) 8 9 U1 = R1 = 100 = U n n + 1) ( = R = = U = 16 (the smaller of the two) Check sums: ( n 1 + n )( n 1 U R n + U n 1 n + R + 1) = = 80 = 8 10 = 80 good, they match = = 171 (8 + 10)( ) = = 171 good, they do too

15 Distribution of the Mann Whitney U statistic (if H 0 is true) A discrete (stepwise) distribution, rather than the continuous distributions we ve looked at before.

16 Determining a significance level (p value) from U If n 0, look up the critical value for U in your tables. (The critical value depends on n 1 and n.) If your U is smaller than the critical value, it s significant (you reject the null hypothesis). If n > 0, the tables don t give you critical values, but by this point the U statistic is approximately normally distributed, so we can calculate a Z score from U and test that in the usual way, using tables of Z. The formula for calculating Z from U is on the Formula Sheet. Our example: U = 16, n 1 = 8, n = 10; critical value of U is 18 (from tables). Our U less than this, so birthweight difference was significant (p < 0.05 two-tailed).

17 Mann Whitney U test: EXAMPLE (a) Transfer along a continuum practical (a previous year s data, I m afraid). Different groups of subjects were trained with {1 or 3} blocks of {easy or hard} discriminations before being tested on similar discriminations. Here are the test scores for the 3-block groups (high = good). Is there an effect of training difficulty? You could use either an unpaired t test or a Mann Whitney U test; try the latter.

18 Mann Whitney U test: EXAMPLE (b) U U Both groups same size. Arbitrarily, call the Easy group group 1 (n 1 = 15). Hard group is group (n = 15). Rank sum R 1 = = 9.5 Rank sum R = = 17.5 n n n n + 1) Last year s (003) data + 1) ( n = 17.5 ( = R = Check sums: ( 1 1 = R1 = = 1 + n )( n 1 U R n + U n 1 n + R + 1) = = = = 5 5 = = 465 U = 5.5 (the smaller of the two) Critical U (n 1 = n = 15) for α = 0.05 two-tailed is 65. So significant. ( )( ) = = good, they match 465 good, they do too

19 Time-saving tip If the ranks do not overlap at all, U = 0. Example: U = 0

20 If you find a significant difference If you conduct a Mann Whitney test and find a significant difference, which group had the larger median and which group had the smaller median? Here, U =. Significant (critical U = 3, α = 0.05 two-tailed). Group 1 has a significantly larger median (even though the rank sums convey the opposite impression). You have to calculate the medians. But this is quick.

21 Two related samples: Wilcoxon matched-pairs signed-rank test Logic Suppose we have a set of n paired scores for each subject, say, we have one score from condition 1 and one score from condition. We can calculate the difference score condition 1 condition for each pair. Then we rank the non-zero differences. If, on average, there is no difference between performance in condition 1 and condition, then the sum of the ranks of the positive differences should be about the same as the sum of the ranks of the negative differences. But if there is a difference between condition 1 and condition, the + and rank sums should differ. Null hypothesis: the distribution of differences between the pairs of scores is symmetric about zero. Since the median and mean of a symmetric population are the same, this can be restated as the differences between the pairs of scores are symmetric with a mean and median of zero.

22 Calculating the Wilcoxon T statistic Easy. From the Formula Sheet: Calculating the Wilcoxon matched-pairs signed-rank statistic, T 1. Calculate the difference scores.. Ignore any differences that are zero. 3. Rank the difference scores, ignoring their sign (+ or ). 4. Add up all the ranks for difference scores that were positive; call this T Add up all the ranks for difference scores that were negative; call this T. 6. The Wilcoxon matched-pairs statistic T is the smaller of T + and T. Check your sums: verify that ( +1) + n T T =.

23 Wilcoxon matched-pairs signed-rank test: EXAMPLE 1 Measure blood pressure (BP 1 ). Make subjects run a lot. Measure blood pressure again (BP ) in the same subjects. Has their blood pressure changed? Sum of positive ranks T + = = 7 Sum of negative ranks T = = 9 Wilcoxon T = the smaller of T + and T = 9. And n = 8. Check sums: T + n+ T = = 36 n( + 1) 8 9 = = 36 good, they match

24 Distribution of the Wilcoxon T statistic (if H 0 is true)

25 Determining a significance level (p value) from T If n 5, look up the critical value for T in your tables. (The critical value depends on n.) If your T is smaller than the critical value, it s significant (you reject the null hypothesis). If n > 5, the tables don t give you critical values, but by this point the T statistic is approximately normally distributed, so we can calculate a Z score from T and test that in the usual way, using tables of Z. The formula for calculating Z from T is on the Formula Sheet. In our example, T = 9 and n = 8. Critical value of T is 4 (for α = 0.05 two-tailed); since our T is not smaller than this, the BP difference was not significant.

26 Wilcoxon matched-pairs signed-rank test: EXAMPLE (a) Proactive interference practical (subset of a previous year s data, I m afraid). Subjects hear and repeat trigram (e.g. CXJ), perform distractor task, recall trigram. Compare trials 9 & 10 (after many similar trigrams) with trials 11 & 1 (after shift to new type of trigram, e.g. 95). Is there release from proactive interference? Note very non-normal difference scores; parametric test unsuitable.

27 Wilcoxon matched-pairs signed-rank test: EXAMPLE (b) Sum of positive ranks T + = (4 8) = 3 Sum of negative ranks T = (4 17.5) + (11 8) = 158 Wilcoxon T = the smaller of T + and T = 3. And n = 19. Check sums: Last year s (003) data T n( + n+ T + 1) = = = = 190 good, they match From our tables, for n = 19 and α = 0.05 two-tailed, the critical value of T is 47. Our T is smaller, so the difference is significant. (In fact, it s significant at α = 0.01 two-tailed.) Subjects did better on trials 11 & 1 than on trials 9 & 10.

28 One sample: Wilcoxon signed-rank test with only one sample Very easy. Null hypothesis: the median is equal to M. For each score x, calculate a difference score (x M). Then proceed as for the two-sample Wilcoxon test using these difference scores. (Logic: if the median is M, then the sum of the ranks of the positive differences from scores where x > M should be the same as the sum of the ranks of the negative differences from scores where x < M. If the median isn t M, then the two rank sums should differ.)

29 Comparison of parametric and non-parametric tests Parametric test Two-sample unpaired t test Two-sample paired t test One-sample t test Equivalent nonparametric test Mann Whitney U test Wilcoxon signed-rank test with matched pairs Wilcoxon signed-rank test, pairing data with a fixed value

30

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