16.9 Is it significant? The one-sample t statistic from a sample of n = 25 observations for the two-sided test of Ho: /X = 64 Ff,: /U 64 has the value t = 1.12.

(a) What are the degrees of freedom for t ?

(b) Locate the two critical values t* from Table C that bracket t. What are the twosided P-values for these two entries?

(c) Is the value t = 1.12 statistically significant at the 10% level? At the 5% level?

16.10 Ancient air, continued. Do the data of Exercise 16.7 give good reason to think that the percent of nitrogen in the air duriiag the Cretaceous era was different from the present 78.1%? Carry out a test of significance, following the four-step process as illustrated in Example 16.3.