A researcher is investigating the effectiveness of a new study-skills training program for elementary school children. A sample of n = 25 third-grade children is selected to participate in the program and each child is given a standardized achievement test at the end of the year. For the regular population of third-grade children, scores on the test form a normal distribution with a mean of = 150 and a standard deviation of s = 25. The mean for the sample is M = 158. What is z?