The probability of a type II error for a hypothesis test

[Mr Davis 97]

Homework Statement

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1. After a long road trip during summer break, a college student wants to estimate how long she drove between rest stops. She takes a random sample of 15 stops and records this data in hours: 1.5, 2.6, 1.75, 3.5, 2.25, 3.75, 1.8, 2.9, 3.2, 3.15, 1.5, 2, 3, 2.75, and 2.2. She thinks that the real mean is not 2.25 hours. Given that the true mean is 2.5 hours, what is the probability that she will make a type II error? What is the power of the test?

Homework Equations

The equation for a t statistic

The Attempt at a Solution

With my calculator, I found that we reject the null hypothesis when t < -3.463 or when t > .7991. I found P(type II error) = .221, and the power of the test to be .779. Is this correct?

 

[statdad]

Why are your critical values not equally spaced either side of 0? What significance level were you using? What work did you do to get the power?