7.50 A watch manufacturer in Switzerland uses sputtered titanium coating for some models it creates. A random sample of nine observations on the thickness of this coating is taken from a week’s output, and the recorded thicknesses (in millimeters) are as follows:

19.5 21.4 18.7 20.3 21.7 19.1 19.5 20.8 20.1 Assuming normality, find a 90% confidence interval for the population variance.