The work of investigators has been the subject of various empirical studies seeking to provide systematic evidence regarding their practices (for example, Greenwood 1970; Bloch and Bell 1976; Greenwood, Chaiken, and Petersilia 1977; Waegel 1981; Eck 1992; Horvath, Meesig, and Lee 2001). These studies have debunked the mythical portrayals of ”super-sleuths” who can solve any crime through careful investigation. In contrast, research shows that most crimes go unsolved and that investigators perform a wide variety of tasks not directly related to this outcome.

Additionally, a number of studies have considered the effectiveness of investigations (for example, Isaacs 1967; Cordner 1989; Eck 1992; Davenport 1999; Wellford and Cronin 2000). Various measures have been used to assess effectiveness, including victim/citizen satisfaction and prosecution results. However, the one most commonly employed is the clearance rate.

The clearance rate is used as a measure for a variety of reasons. Most importantly, it provides direct assessment of the goal of ”crime management”—dealing with crime that has occurred and is reported (Wycoff 1982). This measure also reflects the internal goals of police departments and investigators. As such, this measure is highly valued by practitioners (Horvath, Meesig, and Lee 2001; Davenport 1999). Furthermore, clearance data has been systematically collected through the Uniform Crime Reports (UCR), permitting long-term trend analysis. Virtually every other source of data restricts researchers to cross-sectional analyses.

Defining Clearances

The terminology and requirements for clearing crimes are derived from the guidelines of the UCR program. Clearing a crime occurs in two ways. Most commonly, a crime is cleared by the arrest of one or more suspects. This may occur with or without a warrant. By UCR guidelines, this requires that the arrestee(s) be ”charged with the commission of an offense and turned over to the court for prosecution” (Federal Bureau of Investigation 2004, 255). Alternatively, a crime may be ”exceptionally cleared” when sufficient evidence exists to justify an arrest and prosecution, but the agency is prevented from making the arrest due to circumstances beyond its control. The guidelines require that the exact location of the suspect be known in order to clear a crime by exceptional means.

Clearance statistics are gathered and reported to the FBI regarding the eight Part I offenses: murder and non-negligent manslaughter, rape, robbery, aggravated assault, burglary, larceny, auto theft, and arson. The clearance rate is calculated as the percentage of Part I index crimes reported as cleared by arrest or exceptional means.

Clearance rates are highest for violent crimes (murder, rape, robbery, and aggravated assault). Since 1971, an average of 46.1% of violent crimes have been cleared annually. In contrast, clearance rates for property crimes (burglary, larceny-theft, auto theft, and arson) have averaged 17.5% during that same time period (data drawn from UCR annual reports).

Problems Using Clearance Rates to Examine Performance

Despite the apparent utility of clearance data for measuring investigative effectiveness, the measure is not without its problems. Greenwood, Chaiken, and Petersilia (1977) argued that variations in defining and recording clearances by individual agencies make the clearance rate an inappropriate measure for comparing investigative effectiveness of agencies. Additionally, Sherman and Glick (1984) found that agencies operationally define arrests in various ways, further undermining the reliability of clearance data to examine investigative performance.

In contrast to Greenwood, Chaiken, and Petersilia (1977), Davenport (1999) found that police departments strictly adhered to the UCR coding guidelines for recording clearances. These agencies also indicated that their reporting and coding practices had not changed over time. Thus, it is not clear that interagency variations are necessarily widespread or that they preclude using the data for cross-sectional analyses.

An alternative concern revolves around the potential for agencies to distort their clearance data to reflect either more or less crime solved. It is possible that politically motivated police administrators might manipulate data to secure more funding or improve the appearance of organizational success. However, the concern regarding distortion is more generally applied to reported crime, rather than solved crime, and there is no systematic evidence that it occurs.

Factors Affecting Clearance Rates

Because of the importance of solving crime, researchers have sought to identify the factors that influence clearance rates. Generally speaking, studies focus on environmental factors (underlying community characteristics), contextual factors (event-specific circumstances), and investigative factors (effort and resources).

Because policing occurs in a community context, the characteristics of that environment will have important effects on police performance. Pogue (1975) looked at aggregated clearance rates for metropolitan areas and found that population density had a significant, negative effect. Upon examining clearance data for Part I offenses from a group of Maryland police departments, Cordner (1989) found that geographic region explained most of the variation. Further examination revealed that agencies outside the Washington or Baltimore metropolitan areas had statistically significantly higher clearance rates.

Davenport (1999) used environmental characteristics to analyze Texas clearance data. He found that community complexity and turbulence had modest but significant effects on clearance rates. Both violent and property crime clearance rates were lower where complexity was higher, while turbulence negatively affected property clearance rates. While such factors have only indirect effects on police performance, they clearly constrain the ability of agencies to significantly improve clearance rates.

In the landmark RAND Corporation study, Greenwood, Chaiken, and Petersilia (1977) examined the effects of investigative effort (amount of time spent) on clearance rates. They found that only about 3% of clearances were due to the work of investigators; the vast majority were cleared based on arrests made by patrol officers at the scene or through positive identification of the suspects at the time of occurrence. Thus, they argued that there is no evidence that investigative activities have any substantial effect on clearances. Instead, clearance rates are primarily a function of the circumstances present at the time of the event.

These findings supported the prior research of Isaacs (1967) and Greenwood (1970). Isaacs reviewed a group of case files from the Los Angeles Police Department and found that only 8% of the arrests were due to the activities of detectives. Far more likely was an arrest based on the victim naming the suspect in the initial report. Greenwood considered burglary, robbery, and grand larceny cases in New York City. His research indicated that only 2% of the arrests resulted from investigative follow-up. Most frequently, arrests occurred when a suspect was named by the victim.

However, additional studies have suggested that the relationship between investigative effort and clearances is more complex. Eck (1992) reviewed burglary and robbery cases in three jurisdictions, distinguishing between the information derived from the preliminary investigation and information obtained during the follow-up investigation. Eck hypothesized that if circumstances predicted outcomes, then follow-up activities would not have a significant effect. However, Eck found that both preliminary and follow-up investigatory actions were significant. To further explain these findings, Eck (1992, 103) proposed a ”triage hypothesis,” whereby investigators sort cases based on the strength of preliminary information (solvability factors). Those cases that cannot be solved with reasonable effort receive less activity, as do those cases that have already been solved by circumstances. However, cases that may be solved with reasonable effort (but would not be solved otherwise) receive the most effort. This explanation relies on circumstances for identifying which cases would most likely benefit from activity, but affirms the relative importance of investigative effort for clearing crime.

Further support was provided by Brandl and Frank (1994). They examined burglary and robbery cases for a medium-sized municipal police department and compared the relative effects of two factors: the strength of suspect information emerging from the preliminary investigation and the amount of time spent on the follow-up investigation. For both robberies and burglaries, time spent on cases with moderate suspect information significantly increased the probability of an arrest.

Studies have also focused on clearance rates for homicide, due to the dramatic nature of the crime and the public attention the events receive. Wellford and Cronin (1999) found that various factors, including investigative effort, are important in clearing these crimes. Wellford and Cronin examined more than two hundred factors in 798 homicide investigations from four large municipal police departments. Several case characteristics were significant predictors of clearance status, including factors regarding the victim, the suspect, and general crime circumstances. Detective and investigative variables that influenced clearance status include the number of detectives assigned to the case, time taken to arrive at the scene, and following up on witness information. Consistent with other research, Wellford and Cronin found that information provided by witnesses at the scene has a significant impact on clearance status.

Overall, research has consistently pointed to the importance of witness identifications and cooperation for clearing cases. Though the RAND study (Greenwood, Chaiken, and Petersilia 1977) largely dismissed the role of follow-up investigations, additional research has demonstrated that focused efforts on selected cases produce successful outcomes. It is also clear that careful work at the crime scene (by investigators and responding patrol officers) increases the likelihood of solving crime. Thus, clearance rates are not simply the by-product of circumstances, but are constrained by them in many ways. Investigative effectiveness can be improved, but only within limits created by the context of the criminal event.

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