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

6

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

8

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

9

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

10

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

11

Negative Correlation:Neighborhood Satisfaction and Poverty

43210-1-2

3

2

1

0

-1

-2

-3

-4

Poverty

Neighborhood

Satisfaction

r = - .66

12

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

13

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

14

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

15

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

20

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

22

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