Finding a t critical value
You may need to find a t critical value if you are using the critical value approach to conduct a hypothesis test that uses a tstatistic.
Procedure

ExampleThe US National Research Council currently recommends that females between the ages of 11 and 50 intake 15 milligrams of iron daily. Is there evidence that the population of American females is, on average, getting less than the recommended 15 mg of iron? That is, should we reject the null hypothesis H_{0}: μ = 15 against the alternative H_{A}: μ < 15? The iron intakes (irondef.txt) of a random sample of 25 such American females yielded a tstatistic of 1.48. If we were interested in conducting the test at the α = 0.05 level, what is the appropriate tcritical value to which we should compare the tstatistic? 
Minitab dialog boxBecause the test is leftsided and the desired significance level is 0.05, the appropriate cumulative probability to enter is 0.05. Since there are n = 25 measurements in the sample, the appropriate degrees of freedom is 24. Therefore, your Minitab dialog box should look like: Sample Minitab outputIn this case, Minitab tells us that the tcritical value is 1.7109: 