The Mann-Whitney test is a non-parametric rank test to assess "whether one of two samples of independent observations tends to have larger values than the other" [Wikipedia].

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Kruskal-Wallis Test and Mann-Whitney U Test

I have a three samples of data where each has a non-normal distribution: I want to determine whether the samples differ. Therefore I apply the KW test and it gives me a significant value, i.e. the ...
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How to convert from Mann-Whitney U to Z (or other effect size)?

I'm looking at multiple studies. I do not have the original data. One study I'm looking at provides means, standard deviations, and Mann-Whitney $U$ values (with $p$). (How) can I convert from ...
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51 views

Mann-Whitney test

I am carrying out a study in which I have $3$ groups $n= 2$, $n= 5$, $n=17$. Each group has a different inner ear abnormality, following the fitting of an ear implant I have measured their speech ...
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Unequal variances t-test or U Mann-Whitney test?

I have two independent samples and I need to compare their values, to say if values in the first group are higher than values in the second group. I performed the Levene's test and the variances are ...
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Mann-Whitney U and KS tests in R are not producing significant results [duplicate]

I wish to compare two test data sets using the Mann-Whitney U and KS tests in R. As a start, I thought to compare two similar sets (an A/A test) expecting to see a highly significant indication of ...
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How to convert the wilcox.test W value output for two independent groups to U in R? [duplicate]

I need the U values of Mann-Whitney test, but the wilcox.test in R outputs only W values. I do not think they are identical, so here is my question - how to convert W to U?
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30 views

statistical significance test on cluster analysis result

I've done a hierarchical clustering on a data-set composed by 33 subjects and 2 continuous variables (called V1 and V2), which produces 3 clusters. Now I'm wodering if it make any sense to perform a ...
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Why do we assign fractional ranks in rank tests?

Both Mann-Whitney U test and Wilcoxon signed rank test follow this procedure: sort the data assign ranks, Ties receive a rank equal to the average of the ranks they span compute the statistic using ...
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55 views

Using Wilcoxon-Mann-Whitney test for comparing two population of different sizes

I am using Wilcoxon-Mann-Whitney test for comparing two populations. Unfortunately, sizes of my population are different; one has size 100000 and the other 6000. Can I use this test to compare these ...
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2answers
66 views

How to compare two non-normally distributed samples with very different sizes? (Mann-Whitney vs Randomization/Bootstrap)

Perhaps this is a very basic question, but I didn't find yet a simple solution for this simple problem: I want to compare two samples (say X and Y) for a continuous variable which is non-normally ...
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34 views

What statistical tests can be used on a distribution of distributions?

I have two experimental conditions, each with 20 reps (40 experiments total). In all 40 experiments a non-fixed number of sounds were recorded, and I've measured each of their durations. I would now ...
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63 views

Predictor is significant in logistic regression, but not in Mann-Whitney

I have run a logistic regression using several predictor variables (call them p1 p2 p3 p4) to predict a binary dependent variable (call it y). P1 is a significant predictor of y in the regression. ...
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75 views

What do you do after testing if two non-normal distributions differ?

This is a follow up question to "How do I test if two non-normal distributions differ? I have 13 distributions, . According to the previous question, I should do the following. Kruskal-Wallis test ...
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2answers
231 views

Calculating statistical significance with unequal sample sizes and unequal variances

I have two samples, one with $n_1 = 41,000$ and the other with $n_2 = 881$; the larger sample has a standard deviation of $13.74$, and the smaller has an $SD=10.75$. The means are different, and when ...
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1answer
347 views

How do you report a Mann–Whitney test?

I am doing my dissertation, and I am conducting a number of tests. After using a Kruskal–Wallis test, I usually report the result like this: There is a significant difference ...
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1answer
56 views

Compare Survival in Days using Mann-Whitney-U Test

I already read across the CrossValidated StackExchange, but didn't find an answer on the following problem. I have different groups of animals which have survival rates given in days since first ...
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1answer
30 views

what kind of non-parametric test to use?

I collected data on one sample, the DV could be separated into two groups (success yes vs. no) and then I have several IVs with interval scale. I just don't know if to use Wilcoxon or Man-Whitney ...
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2answers
96 views

Difference Estimate & Confidence Interval for Paired Mann-Whitney Test

I have two paired samples and I am running a paired Mann-Whitney test to determine whether they are different. In Matlab, this takes the form of p = signrank(x,y), which tests the null hypothesis that ...
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30 views

List of sample arrangements at a given Wilcoxon / Mann-Whitney test statistic

Consider a Wilcoxon-Mann-Whitney test statistic, either in the form (i) "W = sum of ranks in sample A"; or in the form (ii) "sum of ranks in sample A - the minimum possible sum of ranks in sample ...
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101 views

Is the W statistic outputted by wilcox.test() in R the same as the U statistic?

I've recently been reading about the Mann-Whitney U test. It turns out that to carry out this test in R you actually need to run a Wilcoxon test! My question: is the W statistic outputted by ...
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2answers
69 views

scale estimate for positive and negative data

I have 2 datasets (nonpaired), P1 & P2, and I want to know 'scale parameter' of one to the other, aka P1=a*P2. As both datasets are highly nonnormal, I am performing a Mann-Whitney Test on the ...
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60 views

Ordinal and longitudinal data

I have repeated measures over time from n subjects responding to a daily Likert scale question, so my data is ordinal and longitudinal over a few months. Not all subjects answer every daily survey of ...
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655 views

Mann-Whitney U test with very large sample size?

I'm doing a Mann-Whitney U test to compare two very large samples (sample size 1 = 13250; samlple size 2 = 38871) originating from a raster image. I know t-tests are not recommended to compare ...
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33 views

Likelihood of a set of ranks

I'm sorry if I cannot formulate the problem precisely. Let us consider an ordered set $Q$ with $n_q$ elements, and a number of its (possibly overlapping) subsets $B_1, \dots, B_k$. For a given $i$, ...
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Difference of 'centers' of 2 non-normal samples with Mann-Whitney test

I have 2 non-normally distributed samples of different sizes (N1~=N2). To evaluate whether there is a significant difference between these samples, I used the Mann Whitney U test (...
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59 views

How to choose the test statistic in Mann-Whitney test?

Reading about the Mann-Whitney test for simple random and independent samples I encountered a small issue. According to the book "Introductory Statistics" by Weiss, the test statistic is obtained ...
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484 views

Large difference between Mann-Whitney test and Wilcoxon signed rank test significance

I am looking at a success of textual requests. We have a dataset of matched pairs, one successful, one not successful, and they are matched based on length of the request in words (since I want to ...
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95 views

Mann-Whitney test on two skewed (Poisson) data sets reject H0 but confidence interval is nearly zero

I'm comparing two sets of samples, X=(x1, x2, ...) and Y=(y1, y2, ...), where xn and yn is an integer (discrete number) from 2-14 denoting the count of a certain event produced by n. The sets differ ...
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266 views

Comparing frequency distributions using Mann-Whitney test

I have two frequency distributions; which each are frequency of an event. I would like to compare these distributions, and conclude that these events and conclude if these events are statistically 1 ...
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140 views

Differences between Dwass-Steel-Critchlow-Fligner and Mann-Whitney U-test for a post-hoc pairwise analysis

If you have one control and four groups of patients with a non-normal small sample size, there are differences in these tests for a post-hoc analysis after Kruskal-Wallis?
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167 views

Running a directional (one-tailed) Mann-Whitney U test in python

As described in this thread, the implementation of Mann-Whitney U provided by scipy doesn't seem to return the direction of the effect (i.e., it's always one-tailed). I other words, it returns if the ...
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91 views

How is power for t test and Mann Whitney comparable when null hypotheses differ

In a previous question, I asked about comparing the power of a t test to a Mann Whitney test under different situations. One of the answers pointed out that the worst that the Mann-Whitney can ever ...
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230 views

Power of a Mann Whitney test compared to a t test

So a Mann Whitney U test is supposedly about 95% as powerful as a t-test when t-test assumptions of normality and homogeneous variance are satisfied. I also know that a Mann Whitney U test is more ...
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148 views

What is the correct way to report the results of independent two sample t test and of Mann Whitney?

I just realised that even though I know how to perform an independent samples t-test or a Mann whitney test, I am not sure how their results should be reported in a paper. I was given this study to ...
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3answers
736 views

Mann-Whitney U-Test critical values for very large samples

I am trying to perform Mann-Whitney U-Test on rather large samples (N=53). Is anybody aware of any resource for critical values for n>30, please? Thank you.
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87 views

Absolute values — parametric or non-parametric test?

I am looking at the absolute difference in values between pairs of numbers. I am then comparing these absolute values between two groups (e.g. Males and Females). Since these are absolute values I ...
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2answers
45 views

What statistical technique would be most appropriate for comparing two instructional approaches?

I work in a school and would like to use a statistical technique for comparing the performance of students using two different instructional programs. A first grade teacher uses a new, specialized ...
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15 views

Five different groups

We are running a study which investigates platelet aggregation changes during three trimesters of pregnancy and postnatal period. Then we compare each of these phases to control non-pregnant women. ...
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946 views

What exactly does a non-parametric test accomplish & What do you do with the results?

I have a feeling this may have been asked elsewhere, but not really with the type of basic description I need. I know non-parametric relies on the median instead of the mean to compare... something. ...
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Un-transformable Seemingly Continuous Data? If not, What to Do With Non-Parametric Test Results - What is the Next Action? [closed]

Note: If you want to skip to the summary of my 3 main questions, they are at the very bottom. I am attempting to produce a binary logistic regressional model for determining whether or not the data I ...
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1answer
191 views

Imputing standard deviations for changes from baseline

This is more of a further question on a previous topic, so my apologies if there was some way to link them that I haven't found. It is essentially the same problem as How to back-calculate change from ...
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180 views

Mann-Whitney for groups filled Likert scale

My stats knowledge is less than basic, so apologies for possible confusion. I have 2 independent groups (N=14 and N=14) who filled in a Likert-scale based standardized questionnaire, consisting of ...
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1answer
99 views

How to bootstrap for the standard error of a two sample test

I want to calculate the standard error of the area under the curve computed with the Mann-Whitney U-Test (MWUT). Hanley and McNeil 1982 give a formula for it (they call the AUC calculated with the ...
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1answer
104 views

Unequal sample size t-test, but with group-effect

I am researching how people watch video lectures together. I have two conditions as follows: (1) 3 groups of 4 learners watch one video lecture on a shared display, but there is only one control (to ...
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496 views

What to do when a Mann-Whitney U assumption is violated?

I'm using a M-W U test to analyse some Likert scale results I have, as my data is ordinal. One of the assumptions that keeps on appearing on references is the following: "both distributions must be ...
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184 views

Two-sample location test when data is heavily skewed towards zero?

I want to test if the mean between two independent samples is different. Both samples are large, about 2 million observations each, however almost all of the observations are zero. In particular in ...
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Test Markov Chain properties of time series

First of all, please excuse if I don't use the proper terminology for this problem. I have a markov chain composed by two states: When in state 1 the output is drawn from an exponential ...
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603 views

Mann-Whitney or t-test to compare age and expenditure after clustering

I have carried out cluster analysis and now want to compare means between variables in different clusters. The variables in question are age and expenditure in millions of dollars. The age variable ...