Introduction to Probation and Parole

Why is it difficult to measure outcomes/successes in correctional programs?
One reference to use is the textbook that we are using which unfortunately I do not have access to upload. I know in the past there have been writers that have had access to the text. Here are the details of the textbook: Latessa, E.J. & Smith, P. (2015). Corrections in the community. New York, NY:
Taylor and Francis. [library e-text]
Post must be at least 250 words