Community corrections DB-200 words

 In June of 2006 a bipartisan committee did a study that found that  parole supervision had as many as 67% of former inmates violate parole  within 3 years after their release. Fifty-two percent of those were  reincarcerated. This costs the American taxpayer as much as 60 billion  dollars. Why is it so difficult to show which correctional programs  reduce recidivism? 

200 words, MUST HAVE IN TEXT CITATIONS FROM REFERENCES LESS THAN 10 YRS OLD.