D. The length of time spent in treatment is the single best predictor of a successful outcome
Explanation:
The success of a treatment is correlated to the length of the treatment period, which is the time spent in treatment
The longer the episode of treatment the increases in the probability of a successful outcome, than the outcome probability obtained in a short period or length of treatment time.
The quality of the treatment program and the candidate motivation are usually related to the retention rates of candidates in a program.
