The Kruskal–Wallis test is a rankbased test that is similar to the Mann–Whitney U test, but can be applied to oneway data with more than two groups.
Without further assumptions about the distribution of the data, the Kruskal–Wallis 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.
It is performed with the kruskal.test function in the native stats package.
Appropriate effect size statistics include maximum Vargha and Delaney’s A, maximum Cliff’s delta, Freeman’s theta, and epsilonsquared.
Posthoc tests
The outcome of the Kruskal–Wallis test tells you if there are differences among the groups, but doesn’t tell you which groups are different from other groups. In order to determine which groups are different from others, posthoc testing can be conducted. Probably the most common posthoc test for the Kruskal–Wallis test is the Dunn test (1964). Also presented are the Conover test and Nemenyi test.
Appropriate data
• Oneway data
• Dependent variable is ordinal, interval, or ratio
• Independent variable is a factor with two or more levels. That is, two or more 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 groups are sampled from populations with identical distributions. Typically, that the sampled populations exhibit stochastic equality.
• Alternative hypothesis (twosided): The groups are sampled from populations with different distributions. Typically, that one sampled population exhibits stochastic dominance.
Interpretation
Significant results can be reported as “There was a significant difference in values among groups.”
Posthoc analysis allows you to say “There was a significant difference in values between groups A and B.”, and so on.
Other notes and alternative tests
Mood’s median test compares the medians of groups.
Aligned ranks transformation anova (ART anova) provides nonparametric analysis for a variety of designs.
For ordinal data, an alternative is to use cumulative link models, which are described later in this book.
Tests similar to Kruskal – Wallis that account for ordinal independent variables are the Jonckheere–Terpstra and Cuzick tests.
Optional technical note on hypotheses for Kruskal–Wallis test
There is a lot of conflicting information on the null and alternate hypotheses for Mann–Whitney and Kruskal–Wallis tests. Some authors will state that they test medians, usually adding an assumption that the distributions of the groups need to be of the same shape and spread. If this assumption holds, then, yes, these tests can be thought of as tests of location such as the median.
Without this assumption, these tests compare the stochastic dominance of the groups. Once a rank transformation is applied, stochastic dominance is exhibited simply by the groups with higher values.
Conover (1999) adds the following assumption:
Either the k population distribution functions are identical, or else some of the populations tend to yield larger values than other populations do.
My understanding is that these tests may yield significant results in the case of stochastic equality and different spread among groups. Conover’s assumption bypasses this difficulty by fiat. The upshot here is one should be careful interpreting these tests when there is stochastic equality and different spreads among groups.
Packages used in this chapter
The packages used in this chapter include:
• psych
• FSA
• lattice
• coin
• multcompView
• rcompanion
• PMCMRplus
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(coin)){install.packages("coin")}
if(!require(multcompView)){install.packages("multcompView")}
if(!require(rcompanion)){install.packages("rcompanion")}
if(!require(PMCMRplus)){install.packages("PMCMRplus")}
Kruskal–Wallis test example
This example revisits the Pooh, Piglet, and Tigger data from the Descriptive Statistics with the likert Package chapter.
It answers the question, “Are the scores significantly different among the three speakers?”
The Kruskal–Wallis test is conducted with the kruskal.test function, which produces a pvalue for the hypothesis. First the data are summarized and examined using bar plots for each group.
Data = read.table(header=TRUE, stringsAsFactors=TRUE, text="
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
Tigger 4
Tigger 4
Tigger 4
Tigger 4
Tigger 5
Tigger 3
Tigger 5
Tigger 4
Tigger 4
Tigger 3
")
### Order levels of the factor; otherwise R will alphabetize them
Data$Speaker = factor(Data$Speaker,
levels=unique(Data$Speaker))
### Create a new variable which is the likert scores as an ordered factor
Data$Likert.f = factor(Data$Likert,
ordered = TRUE)
### Check the data frame
library(psych)
headTail(Data)
str(Data)
summary(Data)
Summarize data treating Likert scores as factors
xtabs( ~ Speaker + Likert.f,
data = Data)
Likert.f
Speaker 1 2 3 4 5
Pooh 0 0 1 6 3
Piglet 1 6 2 1 0
Tigger 0 0 2 6 2
XT = xtabs( ~ Speaker + Likert.f,
data = Data)
prop.table(XT,
margin = 1)
Likert.f
Speaker 1 2 3 4 5
Pooh 0.0 0.0 0.1 0.6 0.3
Piglet 0.1 0.6 0.2 0.1 0.0
Tigger 0.0 0.0 0.2 0.6 0.2
Bar plots of data by group
library(lattice)
histogram(~ Likert.f  Speaker,
data=Data,
layout=c(1,3) # 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 Pooh 10 4.2 0.632 3 4 4 4.75 5 0
2 Piglet 10 2.3 0.823 1 2 2 2.75 4 0
3 Tigger 10 4.0 0.667 3 4 4 4.00 5 0
Kruskal–Wallis 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 ?kruskal.test.
kruskal.test(Likert ~ Speaker,
data = Data)
KruskalWallis rank sum test
KruskalWallis chisquared = 16.842, df = 2, pvalue = 0.0002202
library(coin)
kruskal_test(Likert ~ Speaker, data = Data,
distribution = approximate(nresample = 10000))
Approximative KruskalWallis Test
chisquared = 16.842, pvalue < 1e04
Effect size statistics
Common effect size statistics for the Kruskal–Wallis test include epsilonsquared and etasquared, with epsilonsquared being probably the most common. These two statistics often produce similar results. It’s important to note that both epsilonsquared and etasquared have versions for ANOVA that are distinct from those versions for Kruskal–Wallis. It is therefore important to be sure that an appropriate implementation is chosen. Formulae for these statistics can be found in King and others (2018) and Cohen (2013), respectively.
Technical note: Interestingly, it appears that the epsilonsquared statistic for Kruskal–Wallis corresponds to the etasquared statistic for ANOVA on ranks. However, the definitions presented here appear to be wellestablished.
For Freeman’s theta, an effect size of 1 indicates that the measurements for each group are entirely greater or entirely less than some other group, and an effect size of 0 indicates that there is no effect; that is, that the groups are absolutely stochastically equal.
Another option is to use the maximum Cliff’s delta or Vargha and Delaney’s A (VDA) from pairwise comparisons of all groups. VDA is the probability that an observation from one group is greater than an observation from the other group. Because of this interpretation, VDA is an effect size statistic that is relatively easy to understand.
Interpretation of effect sizes necessarily varies by discipline and the expectations of the experiment. The following guidelines are based on my personal intuition or published values. They should not be considered universal.
Technical note: The values for the interpretations for Freeman’s theta to epsilonsquared below were derived by keeping the interpretation for epsilonsquared constant and equal to that for the Mann–Whitney test. Interpretation values for Freeman’s theta were determined through comparing Freeman’s theta to epsilonsquared for simulated data (5point Likert items, n per group between 4 and 25).
Interpretations for Vargha and Delaney’s A and
Cliff’s delta come from Vargha and Delaney (2000).

small

medium 
large 
epsilonsquared 
0.01 – < 0.08 
0.08 – < 0.26 
≥ 0.26 
Freeman’s theta, k = 2 
0.11 – < 0.34 
0.34 – < 0.58 
≥ 0.58 
Freeman’s theta, k = 3 
0.05 – < 0.26 
0.26 – < 0.46 
≥ 0.46 
Freeman’s theta, k = 5 
0.05 – < 0.21 
0.21 – < 0.40 
≥ 0.40 
Freeman’s theta, k = 7 
0.05 – < 0.20 
0.20 – < 0.38 
≥ 0.38 
Freeman’s theta, k = 7 
0.05 – < 0.20 
0.20 – < 0.38 
≥ 0.38 
Maximum Cliff’s delta 
0.11 – < 0.28 
0.28 – < 0.43 
≥ 0.43 
Maximum 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 
Ordinal epsilonsquared
library(rcompanion)
epsilonSquared(x = Data$Likert,
g = Data$Speaker)
epsilon.squared
0.581
epsilonSquared(x = Data$Likert,
g = Data$Speaker,
ci=TRUE)
### Confidence interval endpoints may vary
epsilon.squared lower.ci upper.ci
1 0.581 0.325 0.805
rsquared for anova on ranks
summary(lm(rank(Likert) ~ Speaker, data=Data))$r.squared
[1] 0.5807692
Ordinal etasquared
library(rcompanion)
ordinalEtaSquared(x = Data$Likert,
g = Data$Speaker)
eta.squared
0.55
ordinalEtaSquared (x = Data$Likert,
g = Data$Speaker,
ci = TRUE)
eta.squared lower.ci upper.ci
1 0.55 0.293 0.794
### Confidence interval endpoints may vary
adjusted rsquared for anova on ranks
summary(lm(rank(Likert) ~ Speaker, data=Data))$adj.r.squared
[1] 0.5497151
Freeman’s theta
library(rcompanion)
freemanTheta(x = Data$Likert,
g = Data$Speaker)
Freeman.theta
0.64
freemanTheta(x = Data$Likert,
g = Data$Speaker,
ci = TRUE)
### Confidence interval endpoints may vary
Freeman.theta lower.ci upper.ci
1 0.64 0.445 0.84
Maximum Vargha and Delaney’s A or Cliff’s delta
Here, the multiVDA function is used to calculate Vargha and Delaney’s A (VDA), Cliff’s delta (CD), and the Glass rank biserial correlation coefficient (rg) between all pairs of groups. The function identifies the comparison with the most extreme VDA statistic (0.95 for Pooh – Piglet). That is, it identifies the most disparate groups.
library(rcompanion)
multiVDA(x = Data$Likert,
g = Data$Speaker)
$pairs
Comparison VDA CD rg VDA.m CD.m rg.m
1 Pooh  Piglet 0.95 0.90 0.90 0.95 0.90 0.90
2 Pooh  Tigger 0.58 0.16 0.16 0.58 0.16 0.16
3 Piglet  Tigger 0.07 0.86 0.86 0.93 0.86 0.86
$comparison
Comparison
"Pooh  Piglet"
$statistic
VDA
0.95
$statistic.m
VDA.m
0.95
Posthoc test: Dunn test for multiple comparisons of groups
If the Kruskal–Wallis test is significant, a posthoc analysis can be performed to determine which groups differ from each other group.
Probably the most popular posthoc test for the Kruskal–Wallis test is the Dunn test. Also presented are the Conover test and Nemenyi test.
Because the posthoc test will produce multiple pvalues, adjustments to the pvalues can be made to avoid inflating the possibility of making a typeI error. There are a variety of methods for controlling the familywise error rate or for controlling the false discovery rate. See ?p.adjust for details on these methods.
When there are many pvalues to evaluate, it is useful to condense a table of pvalues to a compact letter display format. In the output, groups are separated by letters. Groups sharing the same letter are not significantly different. Compact letter displays are a clear and succinct way to present results of multiple comparisons. However, they suffer from presenting “nondifferences” instead of differences among groups, and they obscure the actual pvalues, which are more informative than just a p ≤ 0.05 cutoff.
### Order groups by median
Data$Speaker = factor(Data$Speaker,
levels=c("Pooh", "Tigger",
"Piglet"))
levels(Data$Speaker)
### At the time of writing, it doesn’t appear
that ordering the groups affects
### the dunnTest output.
### Dunn test
library(FSA)
DT = dunnTest(Likert ~ Speaker,
data=Data,
method="bh") # Adjusts
pvalues for multiple comparisons;
# See ?dunnTest
for options
DT
Dunn (1964) KruskalWallis multiple comparison
pvalues adjusted with the BenjaminiHochberg method.
Comparison Z P.unadj P.adj
1 Piglet  Pooh 3.7702412 0.0001630898 0.0004892695
2 Piglet  Tigger 3.2889338 0.0010056766 0.0015085149
3 Pooh  Tigger 0.4813074 0.6302980448 0.6302980448
### Compact letter display
PT = DT$res
PT
library(rcompanion)
cldList(P.adj ~ Comparison,
data = PT,
threshold = 0.05)
Group Letter MonoLetter
1 Piglet a a
2 Pooh b b
3 Tigger b b
### Groups sharing a letter not signficantly
different (alpha = 0.05).
Posthoc tests: Dunn test, Conover test, Nemenyi test, and Dwass–Steel–Critchlow–Fligner test
### Order groups by median
Data$Speaker = factor(Data$Speaker,
levels=c("Pooh", "Tigger",
"Piglet"))
levels(Data$Speaker)
Dunn test
library(PMCMRplus)
DT = kwAllPairsDunnTest(Likert ~ Speaker, data=Data, method="bh")
DT
Pairwise comparisons using Dunn's allpairs test
Pooh Tigger
Tigger 0.63030 
Piglet 0.00049 0.00201
P value adjustment method: holm
library(rcompanion)
DTT =PMCMRTable(DT)
DTT
Comparison p.value
1 Tigger  Pooh = 0 0.63
2 Piglet  Pooh = 0 0.000489
3 Piglet  Tigger = 0 0.00201
cldList(p.value ~ Comparison, data=DTT)
Group Letter MonoLetter
1 Tigger a a
2 Piglet b b
3 Pooh a a
Conover test
library(PMCMRplus)
CT = kwAllPairsConoverTest(Likert ~ Speaker, data=Data)
CT
Pairwise comparisons using Conover's allpairs test
Pooh Tigger
Tigger 0.75549 
Piglet 1.7e05 0.00011
P value adjustment method: singlestep
library(rcompanion)
CTT =PMCMRTable(CT)
CTT
Comparison p.value
1 Tigger  Pooh = 0 0.755
2 Piglet  Pooh = 0 1.69e05
3 Piglet  Tigger = 0 0.000115
cldList(p.value ~ Comparison, data=CTT)
Group Letter MonoLetter
1 Tigger a a
2 Piglet b b
3 Pooh a a
Nemenyi test
library(PMCMRplus)
NT = kwAllPairsNemenyiTest(Likert ~ Speaker, data=Data)
NT
Pairwise comparisons using TukeyKramerNemenyi allpairs test with TukeyDist
approximation
Pooh Tigger
Tigger 0.8912 
Piglet 0.0010 0.0051
P value adjustment method: singlestep
library(rcompanion)
NTT =PMCMRTable(NT)
NTT
Comparison p.value
1 Tigger  Pooh = 0 0.891
2 Piglet  Pooh = 0 0.000996
3 Piglet  Tigger = 0 0.00507
cldList(p.value ~ Comparison, data=NTT)
Group Letter MonoLetter
1 Tigger a a
2 Piglet b b
3 Pooh a a
Dwass–Steel–Critchlow–Fligner test
My understanding is that the Dwass–Steel–Critchlow–Fligner test reranks the data with each pairwise comparison and uses a method to control the familywise error rate of multiple corrections.
library(PMCMRplus)
DSCF = dscfAllPairsTest(Likert ~ Speaker, data=Data)
DSCF
Pairwise comparisons using DwassSteeleCritchlowFligner allpairs test
Pooh Tigger
Tigger 0.7690 
Piglet 0.0012 0.0021
P value adjustment method: singlestep
library(rcompanion)
DSCFT =PMCMRTable(DSCF)
DSCFT
# Comparison p.value
#1 Tigger  Pooh = 0 0.769
#2 Piglet  Pooh = 0 0.00118
#3 Piglet  Tigger = 0 0.00207
cldList(p.value ~ Comparison, data=DSCFT)
# Group Letter MonoLetter
#1 Tigger a a
#2 Piglet b b
#3 Pooh a a
Plot of medians and confidence intervals
The following code uses the groupwiseMedian function to produce a data frame of medians for each speaker along with the 95% confidence intervals for each median with the percentile method. These medians are then plotted, with their confidence intervals shown as error bars. The grouping letters from the multiple comparisons (Dunn test) are added.
Note that bootstrapped confidence intervals may not be reliable for discreet data, such as the ordinal Likert data used in these examples, especially for small samples.
library(rcompanion)
Sum = groupwiseMedian(Likert ~ Speaker,
data = Data,
conf = 0.95,
R = 5000,
percentile = TRUE,
bca = FALSE,
digits = 3)
Sum
Speaker n Median Conf.level Percentile.lower Percentile.upper
1 Pooh 10 4 0.95 4.0 5.0
2 Piglet 10 2 0.95 2.0 3.0
3 Tigger 10 4 0.95 3.5 4.5
X = 1:3
Y = Sum$Percentile.upper + 0.2
Label = c("a", "b", "a")
library(ggplot2)
ggplot(Sum, ### The data frame to
use.
aes(x = Speaker,
y = Median)) +
geom_errorbar(aes(ymin = Percentile.lower,
ymax = Percentile.upper),
width = 0.05,
size = 0.5) +
geom_point(shape = 15,
size = 4) +
theme_bw() +
theme(axis.title = element_text(face = "bold")) +
ylab("Median Likert score") +
annotate("text",
x = X,
y = Y,
label = Label)
Plot of median Likert score versus Speaker. Error bars indicate the 95% confidence
intervals for the median with the percentile method.
Optional technical note: Schwenk dice and pairwise Wilcoxon–Mann–Whitney tests
When choosing a posthoc test, it is often tempting to use pairwise tests, usually with a pvalue adjustment to control the familywise error rate or the false discovery rate. One issue with pairwise tests is that they ignore all the data that isn’t included in that pair of treatments. It is often better to use a posthoc test designed to assess pairs of treatments based on the totality of the data.
In the case of the Kruskal–Wallis test, it is often desirable to use e.g. the Dunn (1964) test that preserves the rankings from all of the original data, rather than to use pairwise Mann–Whitney tests where each test would rerank the data based only on that pair of treatments being compared.
An interesting example of the difference between these two approaches are nontransitive dice. One example is Schwenk’s dice, which are described in Futility Closet (2018) and originally by Schwenk (2000).
Imagine three sixsided dice, which have the following numbers marked on each of the sides. We want to use rankbased statistics to determine which die tends to have higher numbers.
Red = c(2,2,2,11,11,14)
Blue = c(0,3,3,12,12,12)
Green = c(1,1,1,13,13,13)
A simple way to determine which die is stochastically larger is to use Cliff’s delta.
library(rcompanion)
cliffDelta(x=Red, y=Blue)
Cliff.delta
0.167
### The negative Cliff’s delta value suggests the values
in Blue tend to be
### greater than those in Red.
### Specifically, when rolled, the Blue die beats the Red 7 / 12 times.
### ( –0.167 / 2 + 0.5 = 0.4165 = 5 / 12 )
### vda(x=Red, y=Blue) = 0.4165
cliffDelta(x=Red, y=Green)
Cliff.delta
0.167
### The positive Cliff’s delta value suggests the values
in Red tend to be
### greater than those in Green.
### Specifically, when rolled, the Red die beats the Green 7 / 12 times.
cliffDelta(x=Blue, y=Green)
Cliff.delta
0.167
### The positive Cliff’s delta value suggests the values
in Green tend to be
### greater than those in Blue.
### Specifically, when rolled, the Green die beats the Blue 7 / 12 times.
So, in summary, Blue > Red, Red > Green, and Green > Blue. When viewed together, these results would be difficult to interpret, or, honestly, to make sense of.
The same could be shown with pairwise Mann–Whitney tests, if we kept the same data, but increase the sample size. I’ll refer to this new data set as representing three “groups”.
RedBig = rep(c(2,2,2,11,11,14), 25)
BlueBig = rep(c(0,3,3,12,12,12), 25)
GreenBig = rep(c(1,1,1,13,13,13), 25)
wilcox.test(RedBig, BlueBig)
W = 9375, pvalue = 0.01081
wilcoxonZ(RedBig, BlueBig)
z
2.55
### Blue > Red
wilcox.test(RedBig, GreenBig)
W = 13125, pvalue = 0.01038
wilcoxonZ(RedBig, GreenBig)
z
2.56
### Red > Green
wilcox.test(BlueBig, GreenBig)
W = 9375, pvalue = 0.01038
wilcoxonZ(BlueBig, GreenBig)
z
2.56
### Green > Blue
On the other hand, because the Dunn test retains the ranking from all three groups together, it won’t find a difference among any of the groups.
Y = c(RedBig, BlueBig, GreenBig)
Group = c(rep("Red", length(RedBig)), rep("Blue",
length(BlueBig)), rep("Green", length(GreenBig)))
library(FSA)
dunnTest(Y ~ Group)
Dunn (1964) KruskalWallis multiple comparison
pvalues adjusted with the Holm method.
Comparison Z P.unadj P.adj
1 Blue  Green 0 1 1
2 Blue  Red 0 1 1
3 Green  Red 0 1 1
Of course, in this case, the omnibus Kruskal–Wallis test would also find no stochastic differences among the groups.
kruskal.test(Y ~ Group)
KruskalWallis rank sum test
KruskalWallis chisquared = 0, df = 2, pvalue = 1
References
Cohen, B.H. 2013. Explaining Psychological Statistics, 4^{th}. Wiley.
Cohen, J. 1988. Statistical Power Analysis for the Behavioral Sciences, 2nd Edition. Routledge.
Conover, W.J. 1999. Practical Nonparametric Statistics, 3rd. John Wiley & Sons.
Futility Closet. 2018. "Schwenk Dice". https://www.futilitycloset.com/2018/06/27/schwenkdice/.
King, B.M., P.J. Rosopa, and E.W. Minium. 2018. Some (Almost) AssumptionFree Tests. In Statistical Reasoning in the Behavioral Sciences, 7th ed. Wiley.
Schwenk, A.J. 2000. Beware of Geeks Bearing Grifts. Math Horizons 7(4): 10–13.
Tomczak, M. and Tomczak, E. 2014. The need to report effect size estimates revisited. An overview of some recommended measures of effect size. Trends in Sports Sciences 1(21):1–25. www.tss.awf.poznan.pl/files/3_Trends_Vol21_2014__no1_20.pdf.
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 L
1. Considering Pooh, Piglet, and Tigger’s data,
a. What was the median score for each instructor?
b. According to the Kruskal–Wallis test, is there a statistical
difference in scores among the instructors?
c. What is the value of maximum Vargha and Delaney’s A for
these data?
d. How do you interpret this value? (What does it mean? And is
the standard interpretation in terms of “small”, “medium”, or “large”?)
e. Looking at the posthoc analysis, which speakers’ scores
are statistically different from which others? Who had the statistically
highest scores?
f. How would you summarize the results of the descriptive statistics and tests? Include practical considerations of any differences.
2. Brian, Stewie, and Meg 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 among 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
'Meg Griffin' y 3
'Meg Griffin' z 4
'Meg Griffin' aa 3
'Meg Griffin' ab 3
'Meg Griffin' ac 3
'Meg Griffin' ad 2
'Meg Griffin' ae 3
'Meg Griffin' af 4
'Meg Griffin' ag 2
'Meg Griffin' ah 3
'Meg Griffin' ai 2
'Meg Griffin' aj 1
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’s class? (Be sure to report the education level, not just the numeric code!)
b. According to the Kruskal–Wallis test, is there a difference
in the education level of students among the instructors?
c. What is the value of maximum Vargha and Delaney’s A for
these data?
d. How do you interpret this value? (What does it mean? And is
the standard interpretation in terms of “small”, “medium”, or “large”?)
e. Looking at the posthoc analysis, which classes education levels are statistically different from which others? Who had the statistically highest education level?
f. Plot Brian, Stewie, and Meg’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? What do you conclude practically?