A study was conducted to measure the effectiveness of hypnotism in reducing pain. The measurements are centimeters on a pain scale before and after hypnosis. Assume that the paired sample data are simple random samples and that the differences have a distribution that is approximately normal. Construct a 95% confidence interval for the mean of the 'before−after' differences. Does hypnotism appear to be effective in reducing pain? In this example, μd is the mean value of the differences d for the population of all pairs of data, where each individual difference d is defined as the difference in the measurements in centimeters on a pain scale before and after hypnosis. What is the 95% confidence interval for the mean of the 'before−after' differences? Answer using interval notation. (Round to two decimal places as needed.)