 Summary and Analysis of Extension Program Evaluation in R

Salvatore S. Mangiafico

Two-sample Mann–Whitney U Test

When to use this test

The two-sample Mann–Whitney U test is a rank-based test that compares values for two groups.  A significant result suggests that the values for the two groups are different.  It is equivalent to a two-sample Wilcoxon rank-sum test.

Without further assumptions about the distribution of the data, the Mann–Whitney test does not address hypotheses about the medians of the groups.  Instead, the test addresses if it is likely that an observation in one group is greater than an observation in the other.  This is sometimes stated as testing if one sample has stochastic dominance compared with the other.

The test assumes that the observations are independent.  That is, it is not appropriate for paired observations or repeated measures data.

The test is performed with the wilcox.test function.

Appropriate effect size statistics include Vargha and Delaney’s A, Cliff’s delta, among others.

Appropriate data

•  Two-sample data.  That is, one-way data with two groups only

•  Dependent variable is ordinal, interval, or ratio

•  Independent variable is a factor with two levels.  That is, two groups

•  Observations between groups are independent.  That is, not paired or repeated measures data

•  In order to be a test of medians, the distributions of values for each group need to be of similar shape and spread.  Otherwise the test is typically a test of stochastic equality.

Hypotheses

•  Null hypothesis:  The two groups are sampled from populations with identical distributions.  Typically, that the sampled populations exhibit stochastic equality.

•  Alternative hypothesis (two-sided): The two groups are sampled from populations with different distributions.  Typically, that one sampled population exhibits stochastic dominance.

Interpretation

Significant results can be reported as e.g. “Values for group A were significantly different from those for group B.”

Other notes and alternative tests

The Mann–Whitney U test can be considered equivalent to the Kruskal–Wallis test with only two groups.

Mood’s median test compares the medians of two groups.  It is described in its own chapter.

For ordinal data, an alternative is to use cumulative link models, which are described later in this book.

Packages used in this chapter

The packages used in this chapter include:

•  psych

•  FSA

•  lattice

•  rcompanion

•  coin

•  DescTools

•  effsize

The following commands will install these packages if they are not already installed:

if(!require(psych)){install.packages("psych")}
if(!require(FSA)){install.packages("FSA")}
if(!require(lattice)){install.packages("lattice")}
if(!require(rcompanion)){install.packages("rcompanion")}
if(!require(coin)){install.packages("coin")}
if(!require(DescTools)){install.packages("DescTools")}
if(!require(effsize)){install.packages("effsize")}

Two-sample Mann–Whitney U test example

This example re-visits the Pooh and Piglet data from the Descriptive Statistics with the likert Package chapter.

It answers the question, “Are Pooh's scores significantly different from those of Piglet?”

The Mann–Whitney U test is conducted with the wilcox.test function, which produces a p-value for the hypothesis.  First the data are summarized and examined using bar plots for each group.

Input =("
Speaker  Likert
Pooh      3
Pooh      5
Pooh      4
Pooh      4
Pooh      4
Pooh      4
Pooh      4
Pooh      4
Pooh      5
Pooh      5
Piglet    2
Piglet    4
Piglet    2
Piglet    2
Piglet    1
Piglet    2
Piglet    3
Piglet    2
Piglet    2
Piglet    3
")

### Create a new variable which is the Likert scores as an ordered factor

Data\$Likert.f = factor(Data\$Likert,
ordered = TRUE)

###  Check the data frame

library(psych)

str(Data)

summary(Data)

### Remove unnecessary objects

rm(Input)

Summarize data treating Likert scores as factors

Note that the variable we want to count is Likert.f, which is a factor variable.  Counts for Likert.f are cross tabulated over values of Speaker.  The prop.table function translates a table into proportions.  The margin=1 option indicates that the proportions are calculated for each row.

xtabs( ~ Speaker + Likert.f,
data = Data)

Likert.f
Speaker  1 2 3 4 5
Piglet 1 6 2 1 0
Pooh   0 0 1 6 3

XT = xtabs( ~ Speaker + Likert.f,
data = Data)

prop.table(XT,
margin = 1)

Likert.f
Speaker    1   2   3   4   5
Piglet 0.1 0.6 0.2 0.1 0.0
Pooh   0.0 0.0 0.1 0.6 0.3

Bar plots of data by group

library(lattice)

histogram(~ Likert.f | Speaker,
data=Data,
layout=c(1,2)      #  columns and rows of individual plots
) Summarize data treating Likert scores as numeric

library(FSA)

Summarize(Likert ~ Speaker,
data=Data,
digits=3)

Speaker  n mean    sd min Q1 median   Q3 max percZero
1  Piglet 10  2.3 0.823   1  2      2 2.75   4        0
2    Pooh 10  4.2 0.632   3  4      4 4.75   5        0

Two-sample Mann–Whitney U test example

This example uses the formula notation indicating that Likert is the dependent variable and Speaker is the independent variable.  The data= option indicates the data frame that contains the variables.  For the meaning of other options, see ?wilcox.test.

wilcox.test(Likert ~ Speaker,
data=Data)

Wilcoxon rank sum test with continuity correction
W = 5, p-value = 0.0004713
alternative hypothesis: true location shift is not equal to 0

### You may get a "cannot compute exact p-value with ties" error.
###    You can ignore this or use the exact=FALSE option.

Effect size

Statistics of effect size for the Mann–Whitney test report the degree to which one group has data with higher ranks than the other group.  They are related to the probability that a value from one group will be greater than a value from the other group. Unlike p-values, they are not affected by sample size.

Vargha and Delaney’s A is relatively easy to understand.  It reports the probability that a value from one group will be greater than a value from the other group.  A value of 0.50 indicates that the two groups are stochastically equal.  A value of 1 indicates that the first group shows complete stochastic domination over the other group, and a value of 0 indicates the complete stochastic domination by the second group.

Cliff’s delta is linearly related to Vargha and Delaney’s A. It ranges from –1 to 1, with 0 indicating stochastic equality of the two groups. 1 indicates that one group shows complete stochastic domination over the other group, and a value of –1 indicates the complete stochastic domination of the other group.  Its absolute value will be numerically equal to Freeman’s theta.

A common effect size statistic for the Mann–Whitney test is r, which is the Z value from the test divided by the total number of observations.  As written here, r varies from 0 to close to 1.  In some formulations, it varies from –1 to 1.

Kendall’s tau-b is sometimes used, and varies from approximately –1 to 1.

Freeman’s theta and epsilon-squared are usually used when there are more than two groups, with the Kruskal–Wallis test, but can also be employed in the case of two groups.

Interpretation of effect sizes necessarily varies by discipline and the expectations of the experiment, but for behavioral studies, the guidelines proposed by Cohen (1988) are sometimes followed.  The following guidelines are based on the literature values and my personal intuition.  They should not be considered universal.

Optional technical note:  The interpretation values for r below are found commonly in published literature and on the internet.  I suspect that this interpretation stems from the adoption of Cohen’s interpretation of values for Pearson’s r.  This may not be justified, but it turns out that this interpretation for the r used here is relatively reasonable.  The interpretation for tau-b, Freeman’s theta, and epsilon-squared here are based on their values relative to those for r, based on simulated data (5-point Likert items, n per group between 4 and 25).  Plots for some of these simulations are shown below.

Interpretations for Vargha and Delaney’s A and Cliff’s delta come from Vargha and Delaney (2000).

 small medium large r 0.10     –   < 0.30 0.30     –   < 0.50 ≥ 0.50 tau-b 0.10     –   < 0.30 0.30     –   < 0.50 ≥ 0.50 Cliff’s delta 0.11     –   < 0.28 0.28     –   < 0.43 ≥ 0.43 Vargha and Delaney’s A 0.56     –   < 0.64 > 0.34  –   0.44 0.64     –   < 0.71 > 0.29  –   0.34 ≥ 0.71 ≤ 0.29 Freeman’s theta 0.11     –   < 0.34 0.34     –   < 0.58 ≥ 0.58 epsilon-squared 0.01     –   < 0.08 0.08     –   < 0.26 ≥ 0.26

Vargha and Delaney’s A

library(effsize)

VD.A(d = Data\$Likert,
f = Data\$Speaker)

Vargha and Delaney A

A estimate: 0.05 (large)

library(rcompanion)

vda(Likert ~ Speaker, data=Data)

VDA
0.05

library(rcompanion)

vda(Likert ~ Speaker, data=Data, ci=TRUE)

VDA lower.ci upper.ci
1 0.05        0    0.162

### Note: Bootstrapped confidence interval may vary.

Cliff’s delta

library(effsize)

cliff.delta(d = Data\$Likert,
f = Data\$Speaker)

Cliff's Delta

delta estimate: -0.9 (large)

95 percent confidence interval:
lower      upper
-0.9801533 -0.5669338

library(rcompanion)

cliffDelta(Likert ~ Speaker, data=Data)

Cliff.delta
-0.9

library(rcompanion)

cliffDelta(Likert ~ Speaker, data=Data, ci=TRUE)

Cliff.delta lower.ci upper.ci
1        -0.9       -1    -0.67

### Note: Bootstrapped confidence interval may vary.

r

library(rcompanion)

wilcoxonR(x = Data\$Likert,
g = Data\$Speaker)

r
0.791

library(rcompanion)

wilcoxonR(x  = Data\$Likert,
g  = Data\$Speaker,
ci = TRUE)

r lower.ci upper.ci
1 0.791    0.602    0.897

### Note: Bootstrapped confidence interval may vary.

tau-b

library(DescTools)

KendallTauB(x = Data\$Likert,
y = as.numeric(Data\$Speaker))

 0.7397954

library(DescTools)

KendallTauB(x = Data\$Likert,
y = as.numeric(Data\$Speaker),
conf.level = 0.95)

tau_b    lwr.ci    upr.ci
0.7397954 0.6074611 0.8721298

Freeman’s theta

library(rcompanion)

freemanTheta(x = Data\$Likert,
g = Data\$Speaker)

Freeman.theta
0.9

library(rcompanion)

freemanTheta(x  = Data\$Likert,
g  = Data\$Speaker,
ci = TRUE)

Freeman.theta lower.ci upper.ci
1           0.9    0.688        1

### Note: Bootstrapped confidence interval may vary.

epsilon-squared

library(rcompanion)

epsilonSquared(x = Data\$Likert,
g = Data\$Speaker)

epsilon.squared
0.658

library(rcompanion)

epsilonSquared(x  = Data\$Likert,
g  = Data\$Speaker,
ci = TRUE)

epsilon.squared lower.ci upper.ci
1           0.658    0.383    0.842

### Note: Bootstrapped confidence interval may vary.

Optional: Comparison among effect size statistics

The follow plots show the relationship among effect size statistics discussed in this chapter.  Data were 5-point Likert item responses, with n per group between 4 and 25.

Freeman’s theta was mostly linearly related to r, with variation depending on sample size and data values.  In the second figure below, the colors indicate interpretation of less-than-small, small, medium, and large as the blue becomes darker.  The relationship of epsilon-squared and Freeman’s theta was curvilineal, with variation depending on sample size and data values.  In the second figure below, the colors indicate interpretation of less-than-small, small, medium, and large as the blue becomes darker  Kendall’s tau-b was relatively closely linearly related to r, up to a value of about 0.88.  In the second figure below, the colors indicate interpretation of less-than-small, small, medium, and large as the blue becomes darker  References

Cohen, J. 1988. Statistical Power Analysis for the Behavioral Sciences, 2nd Edition. Routledge.

Vargha, A. and H.D. Delaney. A Critique and Improvement of the CL Common Language Effect Size Statistics of McGraw and Wong. 2000. Journal of Educational and Behavioral Statistics 25(2):101–132.

Exercises J

1. Considering Pooh and Piglet’s data,

a.  What was the median score for each instructor?

b.  What were the first and third quartiles for each instructor’s scores?

c.  According to the Mann–Whitney test, is there a difference in scores between the instructors?

d.  What was the value of Vargha and Delaney’s A for the effect size for these data?

e.  How do you interpret this value? (What does it mean? And is the standard interpretation in terms of “small”, “medium”, or “large”?)

f.  How would you summarize the results of the descriptive statistics and tests?  Include practical considerations of any differences.

2. Brian and Stewie Griffin want to assess the education level of students in their courses on creative writing for adults.  They want to know the median education level for each class, and if the education level of the classes were different between instructors.

They used the following table to code his data.

Code   Abbreviation   Level

1      < HS           Less than high school
2        HS           High school
3        BA           Bachelor’s
4        MA           Master’s
5        PhD          Doctorate

The following are the course data.

Instructor        Student  Education
'Brian Griffin'   a        3
'Brian Griffin'   b        2
'Brian Griffin'   c        3
'Brian Griffin'   d        3
'Brian Griffin'   e        3
'Brian Griffin'   f        3
'Brian Griffin'   g        4
'Brian Griffin'   h        5
'Brian Griffin'   i        3
'Brian Griffin'   j        4
'Brian Griffin'   k        3
'Brian Griffin'   l        2
'Stewie Griffin'  m        4
'Stewie Griffin'  n        5
'Stewie Griffin'  o        4
'Stewie Griffin'  p        4
'Stewie Griffin'  q        4
'Stewie Griffin'  r        4
'Stewie Griffin'  s        3
'Stewie Griffin'  t        5
'Stewie Griffin'  u        4
'Stewie Griffin'  v        4
'Stewie Griffin'  w        3
'Stewie Griffin'  x        2

For each of the following, answer the question, and show the output from the analyses you used to answer the question.

a.  What was the median education level for each instructor?  (Be sure to report the education level, not just the numeric code!)

b.  What were the first and third quartiles for education level for each instructor?

c.  According to the Mann–Whitney test, is there a difference in scores between the instructors?

d.  What was the value of Vargha and Delaney’s A for the effect size for these data?

e.  How do you interpret this value? (What does it mean? And is the standard interpretation in terms of “small”, “medium”, or “large”?)

f.  Plot Brian and Stewie’s data in a way that helps you visualize the data.  Do the results reflect what you would expect from looking at the plot?

g.  How would you summarize the results of the descriptive statistics and tests?  Include your practical interpretation.