Nearly 2,700 subjects, between the ages of 15 and 30, had been convicted of 7,716 offenses and served a total of 335 years in prison. By applying the cohort's average conviction frequency per time unit to the time the members were imprisoned, the study results showed that the actual incapacitation effect was around 3 percent. The effect on serious crimes was estimated to be somewhat stronger, between 5 and 6 percent. In this longitudinal approach, all individuals were followed from age 15 to 30; instead of checking whether just one factual conviction would be prevented by a hypothetical prison sentence, the number of convictions prevented were checked and related to the time the offender was at liberty. This procedure was repeated for the first later conviction note prevented by the simulated prison sentence. All estimates of incapacitation effect were based on three assumptions: that offenders were under risk of being detected and convicted, that prevented offenses were not replaced by offenses committed by others, and that prison terms did not influence the convicts' criminal activities after release from prison. 16 tables, 18 notes, and 47 references