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Chapter contents. Nick Tilley, Graham Farrell and Andromachi Tseloni. Introduction. The crime drop:background context
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CHAPTER CONTENTS
· Introduction 230
· The Crime Drop: Background Context 230
· Data Sources, Methods and Methodology 232
¡ Victimization surveys 232
· Findings: Security and the Drop in Car Theft 235
· Security and the Drop in Domestic Burglary 238
· Security and the Drop in Violence between Acquaintances and Strangers 241
· From Reductions in Crime Events to Reductions in Criminality 241
· Contexts, Mechanisms, Security, Crime Drops and Data Signatures 242
¡ Data signatures and alternative explanations of the crime drop 243
¡ Describing and deconstructing the crime drop 244
· Summary and Review 245
· Study Questions and Activities for Students 246
· Suggestions for Further Reading 247
· References 247
GLOSSARY TERMS
methodological tools hypothesis testing clean/dirty data prevalence rates incidence rates repeat crime rates
crime concentration natural experiment data signatures inverse trend causal relationship
DOING QUANTITATIVE DATA ANALYSIS IN CRIMINOLOGICAL RESEARCH
NIck TIlley, Graham Farrell and AndromachI TselonI
INTRODUCTION
This chapter is about quantitative analysis to test hypotheses in criminology. The specific programme of research described addresses a major puzzle in criminology. Against expectations and following a long period of crime increase, why have many types of crime in many jurisdictions fallen steeply?
We describe the data and analytic methods we used to document and try to explain the international crime drop. The chapter begins by introducing the crime drop, ways in which it can be disaggregated, and the hypotheses we developed to try to explain its patterns. Our hypotheses conjectured that security improvements have been both a direct and indirect source of the falls. Next, we say how and why we have used multiple sweeps of large-scale victimization surveys as our main (but not exclusive) data source to test our hypotheses. We then turn to our findings. We show that specific patterns in the crime drop accord with expectations derived from our hypotheses. We describe some analytic tools we developed specifically for our research, notably the Security Impact Assessment Tool, which generates Security Protection Factors. We show how the specific expected detailed data ‘signatures’ within the crime drop correspond to our theoretical expectations, in particular as these relate to vehicle theft and domestic burglary. We go on to discuss what we have done and plan to do to test hypotheses relating to falls in violence and criminality, some of which require greater use of other data sources.
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The chapter concludes by noting that competing explanations for the crime drop
have so far failed to account for detailed patterns within the crime drop, but that there is a rich agenda for future research on the crime drop.
THE CRIME DROP: BACKGROUND CONTEXT
During the roughly 50 years following the Second World War, the best evidence we have from recorded crimes and victimization surveys suggests that the level and rate of crime rose in almost all countries for which data are available. The increases seemed to be inexorable. Then, first in the USA and, after that, in many other coun- tries and against expectations, crime began to fall and to do so precipitously. Moreover, the falls were more than temporary blips: they have been sustained. Since the early 1990s (with variations in specific details), crimes of several types have fallen by more than a half in North America, Europe and Australasia. The changes in direc- tion in crime trajectories took almost all by surprise. They had not been expected by policy-makers, journalists, the general public or, for that matter, criminologists. It seemed to us that the international crime drop poses perhaps the most pressing ques- tions for criminology: what exactly was happening to crime rates and why had they begun to fall?
At the time of writing, we have spent close to a decade working on the crime drop, trying both to describe it more precisely and to explain it.
There are various ways of describing and disaggregating the crime drop and these are detailed in Box 10. 1.
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Ideally, any explanation of the crime drop would make sense of or predict the disaggregated form taken by the crime drop. Indeed, any proposed explanation that is inconsistent with the observed patterns within the overall crime drops thereby becomes suspect. This requirement is akin to that which faced cosmologists looking at the history of the universe and biologists interested in the changing patterns of flora and fauna. In both cases, satisfactory explanations need to be adequate both for the gross observed patterns and for the more detailed expression of those patterns.
The linked set of six hypotheses that has animated the explanatory side of our own work is as follows:
1. The main cause of the crime drop has been increases in security, which have occurred in many countries, and reduced crime opportunities.
2. The improvements in security began with debut crimes, notably theft of cars (which also removed a resource used for many other crimes).
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3. The improvements in security in relation to typical debut crimes inhibited the recruitment of newcomers into criminal careers.
4. The inhibition of the onset in criminal careers has led to widespread drops in diverse crime types.
5. The initial drop in crime has been reinforced by changes in routine activities which have contributed, in particular, to reduced youth crime.
6. Where new opportunities for crime have emerged and where new routine activi- ties have led to encounters with new crime opportunities, numbers of crimes have increased.
We have sought the best data we can find to describe trends as accurately and in as much detail as we can, to evaluate alternative explanations for the crime drop and to test our own hypotheses. This is a huge endeavour, with much remaining to be done. What follows is an account of the methodological approach and tools we have adopted and the forms of analysis we have developed.
In addition to our explanatory interests in relation to the crime drop, we have been committed to teasing out any policy and practice implications of our findings. From the start of our efforts, we took the view that if we could find what was producing the welcome but unexpected crime drops, that ought to be useful in working out what might be needed to maintain or extend them. The security hypothesis, if corroborated in our empirical research, has clear potential signifi- cance for policy.
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