Incomplete and inaccurate data have marred research on the effect of capital punishment on homicides. The most important data problem is that studies have been based on a very incomplete specification of state sanction regimes. Part of the difficulty has been lack of conceptual agreement on how to measure the intensity of use of capital punishment. However, we see the primary problem as a complete absence of data on the noncapital sanctions that might be applied to offenders convicted of homicide. A study of capital punishment in North Carolina by Cook (2009) illustrates the importance of the problem of the absence of information on noncapital sanctions. Of 274 cases prosecuted as capital cases, only 11 resulted in a death sentence. Another 42 resulted in dismissal or a verdict of not guilty, which left 221 cases that resulted in convictions and received noncapital sanctions.

As discussed at length in Chapter 4 and below, there are sound reasons for predicting a correlation between the capital and noncapital components of a state’s sanction regime. Two examples of how this might occur are the plea bargaining leverage that the threat of capital punishment may afford prosecutors and the influence of the state’s political culture on the legislated design and administration of both the capital and noncapital components of the regime. Such a correlation would bias the estimated deterrent effect of capital punishment.

None of the studies we reviewed sought to measure the availability and intensity of use of the noncapital sanction alternatives for the punishment of homicide. Such alternatives may include a life sentence without the possibility of parole, a life sentence with the possibility of parole, and sentences of less than life. It would also be important to have data on the time actually served for convicted murderers who are paroled or who serve less than a life sentence.

It is currently not possible to measure noncapital sanction alternatives at the state level because the required data are not available. The data that are available include those from the Bureau of Justice Statistics (BJS), which publishes nationwide statistics on sentences for prison admissions and time served for prison releases, based on data collected as part of the National Corrections Reporting Program (NCRP) initiated in the early 1980s. More than 40 states now report annual data on sentences for admissions and time served for releases. Individual-level demographic characteristics are also reported. In principle, these data could be used to measure the actual administration of the legally authorized dimensions of most state sanction regimes, not only for murder but also for other types of crimes. The difficulty is that the data are often extremely incomplete.

In some years, states fail to report any data. Just as important, the data that are sent to BJS are often so incomplete that it is impossible to

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