Tagged Questions

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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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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98 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 = z/√(N) This is convenient to me, as I report r also ...
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45 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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20 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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54 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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98 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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323 views

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 using a Welch correction for unequal variances, or by a non-parametric test such as Wilcoxon paired signed rank test, the ...
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18 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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68 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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42 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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1answer
24 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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53 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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106 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
34 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
33 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
162 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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163 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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53 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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305 views

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
80 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
31 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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157 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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203 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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115 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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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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141 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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35 views

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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125 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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705 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 ...
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1answer
115 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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696 views

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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106 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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1answer
53 views

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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1answer
157 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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176 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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38 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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110 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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96 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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2k 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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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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95 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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41 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
336 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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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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475 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
92 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 ...