A random sample of n
8 E-glass fiber test specimens of a certain type yielded a sample mean interfacial shear yield stress of 30.2 and a sample standard deviation of 3.1 (“On Interfacial Failure in Notched Unidirectional Glass/Epoxy Composites,” J. of Composite Materials, 1985: 276–286).

Assuming that interfacial shear yield stress is normally distributed, compute a 95% CI for true average stress (as did the authors of the cited article).