Geog 306, Spring 2008
Lab 6 Problems
April 4/April 11/April 17
Solve these problems twice.
First, calculate the following using R (or by hand), but do
not use any of the “.test” commands.
Then, solve the following problems using the
appropriate test – t.test, wilcox.test, aov.test, oneway.test, kruskal.test, or any other of the
“.test” commands that are correct for the individual problem.
Use both parametric and
nonparametric approaches.
1.
Consumer Reports tested the stopping distances (in feet) of 10 Firestone Winterfire
tires and 12 Michelin XM+S Alpin tires when traveling on ice at 15 mph with the
following results. Can you conclude that there is a difference the mean stopping
distances of the two kinds of tires?
Winterfire: x
1
= 51 ft; s
1
= 8 ft; n
1
= 10
Alpin: x
2
= 55 ft; s
2
= 3 ft; n
2
= 12
2.
Determine whether the numbers of invertebrate species above and below a sewage
outflow are different in a variety of rivers according to this data.
River
Above
outflow
Below outflow
A
8
6
B
9
9
C
12
11
D
8
4
E
15
10
F
7
8
G
14
10
H
5
5
I
7
6
J
11
10
Test to use: t test
Non Parametric: Wilcoxon Signed Rank test
Commands:
di = river$Above  river$Below
di
hist(di)
dbar=mean(di)
dbar
n = length(river$Above)
sd = sqrt( (sum ((didbar)^2)) / (n 1) )
sigma = sd/sqrt(n)
t = dbar/sigma
t
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 Spring '08
 YEO
 representative, Ohio, DI, Nonparametric statistics, Wilcoxon signedrank test

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