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].

learn more… | top users | synonyms

0
votes
0answers
17 views

How do one interpret the results of wilcoxon-mann-whitney test in SAS?

I have done the Wilcoxon test in SAS. My class variable is race. I got a low p-value. What exactly does that mean?
0
votes
0answers
22 views

Statistical test with unequal sample sizes

If I have two samples with vastly different sample sizes (say n1 = 800 and n2 = 21,000) what statistical test would give me the most reliable results? I've been using the Mann Witney U test. Is the ...
1
vote
0answers
19 views

Combining data from different locations to test difference in populations

I have (irregularly spaced) times series concentration data for four locations and am trying to discern whether or not there has been a change in concentrations after a particular event. To avoid ...
3
votes
1answer
95 views

Multiple comparisons after Kruskal Wallis using the FDR approach. How to compute P values (Dunn or Mann-Whitney)?

I want to allow users of GraphPad Prism to use the False Discovery Rate (FDR) approach to multiple comparisons after nonparametric one-way ANOVA (Kruskal-Wallis). The idea is first to compute an exact ...
1
vote
0answers
31 views

For Mann-Whitney, should I report effect size or Hodges-Lehmann pseudomedian?

When reporting a Mann-Whitney-U test result, is it better to report it's effect size or the Hodges-Lehmann pseudomedian (as R give us as "difference in location") ?
1
vote
2answers
24 views

Interpreting contradicting results T-Test vs. Mann-Whitney test (2 independent samples)

I want to test if two indepent experimental groups are sign. different in terms of one outcome variable, but face the following questions: Basic setup 2 groups with 200 observations each Outcome ...
0
votes
0answers
10 views

Comparing Peptide Amino Acid Composition: 2 Independent groups, Non-normal data, discrete data

I have two large groups of peptides, "positive" group (n = 368) and "negative" group (n = 880). The average length of the peptides (in each group) is about 20 amino acids. What I am trying to do is ...
1
vote
2answers
73 views

Relationship between ROC curve and statistical significance in comparison of two groups

My question is about any possible relationship between the significance of a comparison test (Mann-Whitney U) and the ROC curve. If the comparison test is strongly significant, should we expect a ...
0
votes
0answers
16 views

Which test to use: Mann-Whithney U or Spearmans rho or both?

I have the following question: I have a sample group, which is divided in to two subgroups: "good" and "bad" people. I have their points achieved in an IQ test. Points are grouped into 4 categories ...
2
votes
1answer
89 views

chisq.test and wilcox.test don't concord with my graphs and observations?

I want to know if a factor variable have an incidence on another variable (dura). For example : ...
1
vote
2answers
38 views

How to adjust for multiple comparisons with 2 groups where several outcomes were measured?

I have two groups, Controls and Patients, with 6 measurements for each such as speed, amplitude and so on. I'd like to examine whether these measures are different for the 2 groups. So firstly I'd ...
0
votes
1answer
41 views

Similarity between two small datasets

I have gone through several test's (Kolmogorov-Smirnov, t-test, u-test etc..) but I am not sure which one to use when it comes to comparing two small data sets, lets say of size 10 each. ...
1
vote
0answers
24 views

Testing data for Same Shape

I want to run a Mann-Whitney test on 2 arbitrary sets of data but I read that one of the assumptions of the Mann-Whitney test is that the data is the same shape. How can I test if the 2 sets of data ...
0
votes
0answers
20 views

Mann Whitney U test adjustment

Let's say I have 2 separate samples and each sample has 20 observations and I'm using the Mann Whitney U test. As I understand it from wikipedia, I can calculate a z-value that can be converted to a ...
4
votes
1answer
46 views

Why exactly is normality an assumption in a T-test but not a Wilcox rank sum/Mann Whitney U test when obtaining a p-value?

In addition to the question, please do correct anything below that I have incorrect. I understand how the test statistics for a t-test and WRS/MWU test are calculated. In a t-test, $t=\frac{\bar ...
0
votes
0answers
47 views

Which statistical test to use on mean data?

I have sets of data from two sample A and B. the data in sample A and B are the average mean of the population sample. Examle, sample A has list of data. each data in he sample is a an average value ...
0
votes
0answers
48 views

Comparing Likert-scale data to a uniform distrithbution — what size uniform distribution should I use?

Disclaimer: I'm not an expert in statistics, so if I've got something very wrong somewhere please point it out kindly :) I just ran a study and got some 5-point Likert item responses. The study ...
0
votes
0answers
28 views

How to approach a between-subject comparison where only within-subject comparison is valid?

This is a rather odd situation, I'm guessing. I am interested in investigating a between-subjects factor and a within-subjects factor. However, the variable of interest is not one that can be validly ...
2
votes
1answer
72 views

Normality, and when to use t-test vs. Mann-Whitney U-test?

I am looking for a few rules of thumb of when to determine that my data is 'normal enough' to use a t-test vs. a Mann-Whitney U-test. From what I have read, most real world data sets are non-normal, ...
0
votes
0answers
52 views

Choosing right test to compare two populations

I have two populations whose size is 36 and whose values I want to compare. Example ...
0
votes
0answers
120 views

Kruskal Wallis: Effect size

I analyse 4 algorithms and 3 sets of metrics for each algorithm in which I apply the non-parametric Kruskal-Wallis test for each metric to detect any differences in performance between these ...
0
votes
0answers
16 views

Correlations and follow up tests

I am employing non parametric statistics because my research questions are exploratory. I have a mixture of dichotomous variables (age, gender), continous (height, weight), categorical (autism: ...
3
votes
0answers
39 views

Manually computed Mann–Whitney U-test differs from R's output

I found a sketchy lecture notes about Mann–Whitney U-test. There where were given data X: 30 32 33 33 36 37 40 44 Y: 27 29 31 34 34 35 41 It was computed that ...
0
votes
0answers
27 views

Comparison of observations in subgroups?

I have two groups, group A and group B. Each group is composed of subgroups, say A1, ..., An and B1, ..., Bm. Within each subgroup we have a different number of observations, say A1 would contain ...
0
votes
0answers
39 views

Comparing two groups of ordinal survey data?

I am trying to compare two (theoretical) independent groups of surveys with a 5 point ordinal scale (1 - greatly disagree, 5 - greatly agree). Both group A and group B received the same 10 questions ...
2
votes
1answer
93 views

How can a paired test less powerful than unpaired test?

I'm using wilcox.test to compare two timepoints as such: a <- c(1,2,3,4,5) b <- c(2,3,4,5,6) wilcox.test(a, b) The ...
2
votes
0answers
119 views

Reporting Mann Whitney test rejecting the null hypothesis, but samples have equal medians

I'm using the Mann Whitney test to verify if two population samples are equal. If the p-value is less than alpha (in this case, 0.05) I reject the null hypothesis and use the median of each population ...
0
votes
0answers
114 views

Comparing means between two groups if dependent variable is a percentage

I have data on recovery rates from several hospitals across the country. The recovery rates are in percentages. I want to compare the mean recovery rate between two groups of hospitals, one that uses ...
2
votes
1answer
32 views

What test to use to compare these two populations

I have two sets of data. The first one contains the proportion of farmers that own land in 22 different towns. The other contains the proportion of craftsmen that own land in the same 22 towns. I ...
3
votes
2answers
171 views

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 $n$ ...
1
vote
1answer
77 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 ...
1
vote
1answer
77 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 ...
5
votes
2answers
203 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 ...
8
votes
1answer
674 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 ...
2
votes
0answers
70 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 ...
2
votes
1answer
56 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 $n_1=6$ ...
2
votes
1answer
136 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 ...
0
votes
0answers
46 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 ...
3
votes
2answers
257 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 ...
1
vote
0answers
31 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 ...
0
votes
1answer
20 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 ...
2
votes
1answer
110 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 ...
0
votes
0answers
11 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 ...
3
votes
1answer
740 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 ...
1
vote
1answer
28 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 ...
0
votes
1answer
88 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 ...
2
votes
0answers
62 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 ...
2
votes
1answer
163 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?
2
votes
1answer
376 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 ...
2
votes
2answers
2k 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, ...