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RELATIONSHIP BETWEEN THE OFFENDING FREQUENCY (A) OF IMPRISONED AND FREE OFFENDERS*
In the public debate over incarceration policy, there is considerable disagreement about what value of individual offending frequency (A) is appropriate to use in estimating incapacitative effects. This article provides an approach for estimating the mean values of A for diverse subsets of the total offender population, with particular emphasis on subsets generated by filtering through various stages of the criminal justice system. Sharp differences in offending frequency are displayed between robbery and burglary inmates, across three states, and particularly between resident inmates and free, active offenders. Free offenders average I to 3 robberies and 2 to 4 burglaries per year, while resident inmates have A values 10 to 50 times higher. Differences result from the underlying levels of criminal activity and the sanction levels that offenders face. A highly heterogeneous distribution of offending frequency in the total population of offenders combines with relatively low imprisonment levels to lead to substantial selectivity of high-lambda offenders among resident inmates and a correspondingly low mean value of A among those offenders who remain free. These results have important implications for estimating incapacitative effects of an increase in incarceration, since the additional inmates will be drawn from free offenders whose mean A is at least an order of magnitude lower than that of the current inmate population.
ESTIMATING OFFENDING FREQUENCIES IN FILTERED POPULATIONS
Measuring the frequency of offending by active offenders is an extremely important issue in criminological research. Following its introduction in some pathbreaking models proposed by Avi-Itzhak and Shinnar (1973) and Shinnar and Shinnar (1975), measurement of individual offending frequencies emerged as central in understanding the nature of criminal careers (Blumstein and Cohen, 1979; Chaiken and Chaiken, 1982; Blumstein et al., 1986; Horney and Marshall, 1991) and in policy considerations, especially the magnitude of incapacitative effects (Blumstein et al., 1978; Cohen, 1978, 1983, 1984; Spelman, 1994; Zimring and Hawkins, 1995).
Offending frequency refers to the rate of crimes by offenders who commit crimes at some nonzero rate, and is distinct from the more conventionally used population incidence rates obtained from the ratio of total crimes to total population (Blumstein et al., 1986). Studies of offending frequency thus require samples of active offenders, and for reasons...