1 Introduction to Research Methods How we come to know about crime.
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Transcript of 1 Introduction to Research Methods How we come to know about crime.
1
Introduction toResearch Methods
How we come to know about crime
2
Why Study Crime?
• To learn about the causes of crime
• Make predictions about criminal behavior
• Provide information to policy makers
3
The Desire to Generalize
• Goal: External Validity– We seek knowledge that is generalizable to
some larger group
• Method: Random sampling– Yields a representative set of observations– Each member of the population has an equal
chance of being measured
4
Statistical Significance
• A result is statistically significant if the probability that it is due to chance is less than 5% (α = .05), p < .05
5
What Is a Variable?
Variable Attributes/Valuesyoung, middle-aged, old
male, female
plumber, lawyer, professor
Race
Social Class
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Describing VariablesMeasures of Central tendency
Age Number
13 X X XX X XX
14 X XX
15 X XX
16 X X X X X X X X
17 X X X X
18 X
Mode = 16
Median = 15.5
Arithmetic Mean (Average) = (ΣYi)/N = 392/26 = 15.07
13
13
7
Two Kinds of Variables
• Dependent variable: what we wish to explain or understand (crime, violence)
• Independent variable: what we think explains the dependent variable (poverty, age, associating with other criminals)
Independent variable Dependent Variable
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Temporal Order
Research Designs
• Cross-Sectional: Gather data all at once
• Longitudinal: Gather data over time to ensure proper causal ordering
Unemployment Property Crime
Property Crime Unemployment
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Measures of Association:Correlation
• Do values on one variable correspond to values on another variable?
• If no, correlation = 0
• If yes– Correlation = +1.0– Correlation = -1.0
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Positive Correlation:Poverty and Homicide in Chicago
Poverty
43210-1-2
2.0
1.5
1.0
.5
0.0
-.5
-1.0
-1.5
Hom
icide R
ate
r = .51
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Negative Correlation:Neighborhood Satisfaction and Poverty
43210-1-2
3
2
1
0
-1
-2
-3
-4
Poverty
Neighborhood
Satisfaction
r = - .66
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No Correlation:Social Ties and Poverty in Chicago
43210-1-2
5
4
3
2
1
0
-1
-2
-3
Poverty
Social
Ties
r = .02
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Crosstabulation:Homicide by Poverty in Chicago
Low Poverty
High Poverty Total
Low Homicide
109
56.8%
19
12.7%
128
37.4%
High Homicide
83
43.2%
131
87.3%
214
62.6%
Total 192 150 342
Positive Relationship
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Neighborhood Satisfaction byPoverty in Chicago
Low Poverty
High Poverty Total
Low Satisfaction
55
28.6%
123
82.0%
178
52.0%
High Satisfaction
137
71.4%
27
18.0%
164
48.0%
Total 192 150 342
Negative Relationship
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Social Ties by Poverty in Chicago
Low Poverty
High Poverty Total
Low Homicide
101
52.6%
75
50.0%
176
51.5%
High Homicide
91
47.4%
75
50.0%
166
48.5%
Total 192 150 342
No Relationship
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WARNING:Correlation = Causation
Ice Cream Murder
Season
Ice Cream Murder
“Spuriousness”
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Criteria of Causality• Temporal order• Covariation• No plausible alternative explanation
– Association between two variables is not the result of some other variable causing them both
– The relationship between the independent and dependent is not spurious
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Drawing Causal Diagrams
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Data Gathering Methods
• Survey Methods
• Field Observation (ethnography)
• Unobtrusive Measurement
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Gender Discriminationat XYZ University?
Males Females Total
Accepted 82
75%
28
25%
110
50%
Denied 28
25%
82
75%
110
50%
Total 110 110 220
XYZ University has 2 colleges: Liberal Arts and Engineering
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College of Liberal Arts
Males Females Total
Accepted 2
20%
20
20%
22
20%
Denied 8
80%
80
80%
88
80%
Total 10 100 110
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College of Engineering
Males Females Total
Accepted 80
80%
8
80%
88
80%
Denied 20
20%
2
20%
22
20%
Total 100 10 110
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Conclusion• There is no gender discrimination at XYZ!
• Women tend to apply to departments that have higher rejection rates
• Moral of story: control variables
• Simpson’s paradox
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Experimental Research Design
Experimental Group Control Group
Measure DV Measure DV
RANDOM ASSIGNMENTRANDOM ASSIGNMENT
Test
Remeasure DV Remeasure DV
Compare
Compare
The Scientific Study of Crime
TheoryWritten explanation of why
things happen
Observation
Scientific methods for observing what happens
Empirical HypothesesGeneralizations