Police departments have generated and used MO files since the turn of the 20th century. MO files provide information on an offender's method of operation during the preparation for and commission of a criminal act, as well as evidence of the offender's behavior after the crime has been committed. A central issue in the usefulness of MO files in criminal investigations is the probability that an offender in a new case will be in the MO file, as well as the sensitivity of that probability to a variety of factors. Reliance on MO files should be greater with increases in the probability that an offender in a new case is in the MO files. That probability will be inversely related to the percentage of offenders who never get arrested, and in turn on the effects of each offender's stealthfulness, the rate at which offenders learn to avoid arrest as their criminal experience increases, and the frequency of offending. A simulation model provides a basis for making such estimates for stranger rape cases. Key parameters were the average number of offenses per offender, the aggregate arrest rate, inherent stealthfulness, and increases in stealthfulness as the number of offenses increased. 3 tables, 7 notes, and 38 references