How to find p-values - AP Statistics

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Question

We are testing the hypothesis that the average gas consumption per day in Billings, Montana is greater than 7 gallons per day; we want 95% confidence.

We sample 30 drivers. The average is 8.4, and the sample standard deviation is 4.29.

Our null hypothesis is

  1. What is the Z-value for a 1-tailed test at 95%?

  2. What is the Z-value for our sample mean of 8.4?

  3. What is the p-value for our sample mean of 8.4?

  4. Do we reject the null hypothesis?

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Answer

From the Z-table: 1.8 corresponds to .9641; p= 1 - .9641 = .036

We reject the null hypothesis since 1.8 > 1.64 (and .036 is less than 95%).

In plain English, we are 95% sure that we will not get a sample mean of 8.4 when the true population mean is 7.

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