twenty people employed in the same industry took the inventory risk survey catalog (irisc) and a promblem solivng test. The promblem solving test presented 20 difficult word promblems, and the score is the number of promblems the person solved. their ages (in years) and annual salaries (in dollars) were also provided. these fourvariables were analyzed by pearson correlations. with a sample size of 20, how many degrees of freedom apply to eavh bivariate correlation