I'm looking over corrections for one of my midterms and ran into a few problems giving me weird Z-values from confidence intervals that don't exactly correspond to my Cumulative T Distribution Table.

Question

How do I calculate the Z value required? The answer it gives me is Za/2=1.44 but I have no clue why and can't find "Za/2" anywhere in my notes to determine how all of this works.A sample of 19 eggs of the Atlantic Fairy Tern are each measured. The mean diameter is 3.34 cm. The actual population standard deviation of the diameter is 0.2 cm, what is the lower limit of an 85% confidence interval on the mean?

Thanks!