Car gets mean of 18mpg. Test see if cheaper grade of gasoline made difference in gas miles at .01 level. Sample of 38 fill ups with cheaper gas got mean of 17.8mpg and standard deviation of 0.6. State hypotheses and identify claim, find critical value(s), compute test value, made decision, summarize results.

Solution: The hypotheses are:

We use a left-tailed t-test (with normal approximation) to test the null hypothesis. We claim that the cheaper gas gets lower mileage. Using Statdisk we obtain

The test statistics is equal to -2.0548, and the critical value is -2.3263. That means that we fail to reject the null hypothesis at the 0.01 significance level. In other words, we don’t find enough evidence in the sample to support the claim.