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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Mann-Whitney U test and paired data

I've read that working with paired data requires other method than Mann-Whitney $U$ test. I am not sure about how formally accurate is this statement. Reasons: Suppose that there is a set of $n$ ...
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43 views

Testing for mean statistical difference in segments of different length in the same Time Series

I have a Time Series of 336 monthly observations, from January 1987 to December 2014. I'd like to test for differences in uneven (of different size) segments of this time series. In effect, it would ...
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41 views

3 values ordinal variable: Mann-Whitney or Chi-squared?

I have an ordinal variable which assumes only 3 values. I need to do a comparison between 2 treatments. I used Mann-Whitney test, since is an ordinal. My "p-value" is quite high (0.9388) which is ...
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24 views

Reporting Results of Mann-Whitney U Test - Means vs Medians

I had a question yesterday that I got a bunch of great responses on. I have, hopefully, a quicker question here. There is a debate in the literature regarding reporting means vs medians in a ...
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161 views

Displaying Ordinal Data - Means, Medians, and Mean Ranks

Thanks in advance for any help. I have some ordinal data that is not normally distributed, so I decided to do non-parametric testing using the Mann-Whitney U Test. I am looking at differences ...
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29 views

Hypothesis testing with non-iid observations

I have a dataset with observations which violate the independent and identically distributed (iid) assumption. The dataset is composed by a number of post-click conversion events (purchases on a ...
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1answer
30 views

Finding the P Value using the Mann Whitney Test

The problem is given below Along with the picture of the ranks ![enter image description here][2] I let the first sample be $X$ and the sum of the ranks which I let $W$ be is 44. Also $n_1=6$ ...
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1answer
57 views

Mann Whitney test, unequal sample sizes, different shaped distributions

I was hoping someone could help, I am comparing proportional data in to two different groups. One group has a sample size of 22 and the other 530. The data are not normally distributed and have ...
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28 views

Mann Whitney or t test?

I have 4 questions which ask different branches of the same topic (I suppose they are related to each other). Different numbers of people responded to each question. They responded to each question by ...
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148 views

Mann-Whitney U test with unequal group sizes and one group very small

I have a sample of 24 participants who gave their responses to 8 questions. I calculated their mean response. Can I use Mann Whitney to compare between 2 groups where one of them has only 2 ...
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21 views

Is it okay to convert binary data to percentage data?

I have a within-groups study with 4 groups (400+ users). Each user answered 10 questions for each group. The answers were scored as right/wrong (1/0). Can the percentage correct for each user/group be ...
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1answer
17 views

How to combine clinical variables?

I am dealing with several clinically used variables that are not linked to each others. Those variables are correlated to worsened prognosis (some are numeral and asymmetric, therefore correlations ...
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1answer
65 views

Mann-Whitney U test when only summary data (mean, sd, sample size) are available

When raw data is not available, which normally occurs when we obtain our data from different scientific papers, how is it possible to perform a Mann-Whitney analysis to test whether two samples are ...
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10 views

Dealing with gender bias in Clinical Field Study - non parametric data

I have 105 patients to compare to 191 non-patients, across numerous non-parametric variables. However, my key confound is gender: 60% male in the patient group and 35% male in the non-patient ...
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1answer
152 views

Kolmogorov-Smirnov vs Mann-Whitney U When There Are Ties

I have a dataset consisting of rank data, some 100 cases and 2 groups. (The 2 groups contain about 1/3 and 2/3 of the cases.) I would like to test whether the two groups differ with respect to median ...
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1answer
20 views

Mean comparison of two categorical variables

I'd like to compare people's perception of safety at a certain location [which is a categorical variable on a scale of 1-10, with 1=feel very unsafe & 10=feel very safe] before and after being ...
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1answer
66 views

multiple pair wise nonparametric comparisons

is it possible to perform multiple pair wise nonparametric comparisons (generally using a Mann–Whitney test) and adjust the significance level used for the decision criteria using a Bonferroni-type ...
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46 views

Am I using Paired Statistical Test Correctly?

I am comparing a feature between correct & incorrect trials. There are N=1194 paired samples $\{x_i,y_i| i=1,2,...N\}$, where x is for correct trials and y is incorrect trials. I am using a ...
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1answer
131 views

Does the Mann-Whitney U-test require the groups to have the same distribution?

If one of my data sets is normally distributed and the other is not, can I do a Mann-Whitney U-test on them, or would they both have to be non-normal?
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1answer
73 views

How do I interpret the p value returned in scipy's Mann Whitney U test?

I have a program in which I need to perform an action based on what results of a Mann Whitney U test are. I know the criteria that need to be met, but I would still like to understand what the results ...
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1answer
460 views

Mann-Whitney U-test: confidence interval for effect size

According to Fritz, Morris, and Richler (2011; see below), $r$ can be calculated as an effect size for the Mann-Whitney U-test using the formula $$ r = \frac{z}{\sqrt N} $$ This is convenient to me, ...
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1answer
53 views

Using nonparametric tests with small samples even when data are normaly distrubuted

Is it appropriate to use a nonparametric test with a small sample (e.g., less than 65 people) even when data are normally distributed? For example, using the Spearman correlation? Actually my data is ...
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35 views

Hypothesis testing on groups of time series data

I have some experimental data from two groups, where each group contains data from $n$ subjects. The data are in the form of a time series for each subject, but are not all the same length. To be ...
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1answer
71 views

Why are there two forms for the Mann-Whitney U test statistic?

I have encountered two forms for calculating the two-sample Mann-Whitney U test statistic, which are: $$U_1 = R_1 - \frac{n_1(n_1 + 1)}{2}$$ and $$U_1 = n_1n_2 + \frac{n_1(n_1+1)}{2} - R_1$$ where ...
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1answer
161 views

Significant p value for Mann-Whitney U test but low AUC

How is it possible that for two sample sets I'm getting a low p-value, but also a low AUC value (just below 0.5)? To compute the P-value I'm looking at the second outputted value of the function here ...
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A principled method for choosing between t-test or non-parametric e.g. Wilcoxon in small samples

Certain hypothesis tests can be approached using Student's t-test, possibly with a Welch correction for unequal variances in the two-sample case, or by a non-parametric test such as Wilcoxon paired ...
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1answer
56 views

power to detect effect sizes for the usual t, Welch Adjusted t, and Mann-Whitney U test

For class, I am given a series of sample sizes and variances for two groups and am asked to calculate the power for detecting effect sizes for the usual t test, Welch adjusted T test, and the ...
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91 views

Best way to compare pre vs post levels in treatment vs control groups?

In a study, drug was given to tt (treatment) group and not to cc (control) group and blood levels of (say) cholesterol were tested before and after in each subject. I have following data: ...
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63 views

How to apply Mann-Whitney test Control vs Treatment Group

I have a treatment and a control group: both have pre- and post- values (I tested reading speed). I need to know if there are any significant differences between the groups (or if the treatment group ...
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34 views

If I am comparing actual data vs. forecasted data, is the Durbin-Watson statistic useful?

We are comparing our forecast vs actual data from the same time period and was using the mann-whitney test to help provide evidence our samples were not different. Someone challenged us saying the ...
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1answer
277 views

Problem with Mann-Whitney U test in scipy

I am trying to perform Mann-Whitney test on a real data set using scipy. I extracted an example where the problem occurs. It seems that the function tiecorrect returns negative value which is the ...
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2answers
90 views

Mann-Whitney/Wilcoxon rank sum test in R with a table of tallies

I have a 10 treatments, 5 samples per treatment, and the result of each treatment is ranked on a ordinal scale: Poor, Fair, Moderate, Good, Excellent. I have the data tallied up so I have the total ...
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172 views

Mann-Whitney U-test in R

I'm doing a Mann Whitney U test in R but it won't run the code: downstream<-subset(mydata,Direction=="D") Wilcox.test(Date, Species, data="downstream") ...
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1answer
37 views

How to show whether the average coefficients of determination from one regression technique are better than another across many objects?

I have 50,000 objects on which I have performed two different types of regression. Using cross validation, I obtained the average $R^2$ score from each model on each of the objects. So now I have a ...
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1answer
35 views

wilcox.test usage in R

I would like to know if there is a difference in the values taken from measurements of a given characteristic sampled at primary forest (Point=1) and secondary ...
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2answers
263 views

Mann Whitney test with unequal variances [duplicate]

I am confused on what I have read about the Mann whitney test. We are testing whether our actual data is the same as our projected data. We had been using the t-test until we realized the data might ...
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1answer
303 views

Non-parametric version of paired t-test (Mann–Whitney U test)

I have data from 100 individuals doing a 2AFC under a control and a manipulated setting. I want to compare the means. I initially used a paired t-test. My supervisor noticed the distributions were not ...
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1answer
61 views

Relative increase between two pairs of samples

Consider the following samples from four distributions: $a = \{20, 2, 200\} \qquad\qquad c = \{1, 10, 100\}$ $b = \{22, 2.2, 220\} \qquad\quad\:\,d = \{1.2, 12, 120\}$ I would like to say that the ...
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Can Mann-Whitney test be used for post-hoc comparisons after Kruskal-Wallis?

I have a simulation where an animal is placed in a hostile environment and timed to see how long it can survive using some approach to survival. There are three approaches it can use to survive. I ran ...
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1answer
110 views

Reporting Mann-Whitney U-Test without homogenity of variance

I am really stuck with this question: The Mann-Whitney test requires homogenity of variance if a median difference is suppossed to be statistically significant. In case homogenity of variance is not ...
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1answer
32 views

Understanding Regression vs. Means/Median Results

I am having a little difficulty understanding my results - could someone help me understand how to interpret, and if my process is sensible? Here is an example of what I am doing I am trying to ...
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179 views

Mann whitney U test

I would like to compare the nutrient intake of males with that of females. One of the assumptions of the Mann Whitney test U test is that the two variables must not be normally distributed. The ...
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2answers
314 views

Kruskal–Wallis Test not significant but Mann Whitney U significant

I have a non-normalized dependent variable and an independent variable broken down into 4 groups. As such, I used the Kruskal–Wallis analysis to look for significant differences in the ranks of the ...
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175 views

Comparing ROC-curves

I would like to find if there is a significant difference between two ROC-curves. I've found the roc.test in the pROC package. However, I cannot seem to find any information on how this test is ...
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23 views

statistical method

I want to compare time spent in shopping between two group people. One group has 1426 cases, whereas the other group only has 96 cases. Which statistical method should I use, t-test or Mann-Whitney ...
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2answers
86 views

I don't understand what is going on with my t-test and Mann-Whitney results

I have 2 samples, A and B. I subjected both samples to extraction and got the amount of yield (the dependent variable). I just want to see if there is a significant difference between the yield for ...
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1answer
208 views

Mann-Whitney U Test Analysis Help

Good day! I'm totally new to SPSS (and the test as well, but after some research I've finally demystified a bit about Mann-Whitney and how to do the test in SPSS. The only problem I'm currently facing ...
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Mann-Whitney or ANOVA?

I am conducting a randomized vignette survey and would like to know if Mann-Whitney is the best test to use . Participants will be randomized to an identical scenario where a patient deteriorates ...
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193 views

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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1answer
1k views

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 ...