Ellis and colleagues (2012) tested a new psychotherapy on depression. To study this, a sample of N = 20 inpatients at a psychiatric hospital completed a battery of measurements before and after treatment. Specifically, the sample rated their sense of hopelessness on the Beck Hopelessness Scale (BHS), where the lower the score, the less helpless the patient feels. Feelings of hopelessness are one major symptom of depression. Once psychotherapy was completed, the difference between before and after treatment was calculated, and the sample had M = -5.34 on hopelessness. Ellis et al. decided to re-conduct the two-tailed t test now with a more rigorous 0.01 significance level. They used the same values as before: t = -2.62 for the sample mean and d = 0.83.1. What is the degrees of freedom (df)