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±
Variability (Chapter 4)
±
Types of measures
±
Computing measures
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Choosing you measure
0
11.25
22.50
33.75
45.00
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Frequency
Score
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10
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30
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Score
M=6 for both distributions.
Would it be fair to say they were the same?
The Three Stooges®, characters, and al related indicia are trademarks of C3 Entertainment, Inc.
±
On 5 different occasions
± Moe: 30 min. late every time
± Curly: 25, 35, 28, 32, 30 min. late
± Larry: On time, 60, 30 min early, 90 min late, 30 min late
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All average 30 min. late
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The point: if you were waiting for one of them, the average wouldn²t help
you out much in deciding how long you should wait
±
This is where variability comes in
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Is a quantitative measure of the amount of
spread of the scores in a distribution.
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Provides a measure of consistency within a
distribution
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Can be used to determine how well a
single score (or set) represents the
distribution
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Can be used to estimate how well a sample
characterizes a population
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The distance between the lower real limit of the
minimum score and the upper real limit of the
maximum score
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123456789
scores
frequency
RANGE
Not the range
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How to get it:
1.
Get the largest score (X
max
)
2.
Get the smallest score (X
min
)
3.
Get real limits for X
max
and X
min
4.
Range =
X
maxURL
 X
minLRL
± Example:
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Scores: 5, 7, 9, 12, 16
±
X
maxURL
= 16.5
±
X
minLRL=
4.5
± Range = 16.5  4.5 = 12
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IQR:
Distance between the
rst and third quartile.
±
First Quartile:
Score that separates the lowest 25 %
percent of the scores (25 percentile).
±
Third Quartile:
Score that separates the lowest 75 %
percent of the scores (75 percentile).
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This note was uploaded on 02/18/2011 for the course PSYCH 5 taught by Professor Collins during the Fall '08 term at UCSB.
 Fall '08
 COLLINS

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