A manufacturer bonds a plastic coating to a metal surface. A random sample of nine observations on the thickness of this coating is taken from a week's output, and the thicknesses (in millimeters) of these observations are as follows:
19.8 21.2 18.6 20.4 21.6 19.8 19.9 20.3 20.8
Assuming normality, find a 90% confidence interval for the population variance.