A topic of interest in ophthalmology is whether or not spherical refraction differs between the left and right eye on average. In a study to investigate this, refraction was measured on the left and right eye of 17 patients. The average of the differences (right - left) in diopters was -3.5 with a sample standard deviation of 7.5. Assuming normality, provide a 90% confidence interval for the average difference (right - left)