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# 8.3 t- test for a mean

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1. 8.3 t- test for a mean

2. T- Test • The t test is a statistical test for the mean of a population and is used when the population is normally or approximately normally distributedand is unknown. • The formula for the t test is • The degrees of freedom are d.f. = n – 1.

3. Finding critical values • Find the critical t value for α = 0.05 with d.f. = 16 for a right-tailed t test.

4. examples • 1. Find the critical t value for α = 0.01 with d.f. = 22 for a left-tailed test. • 2. Find the critical value for α = 0.10 with d.f. = 18 for a two-tailed t test.

5. T-test steps • Use the same steps as for the z-test. • State null and alternative hypothesis. • Find the critical values from Table F. • Compute the test value. • Reject/Not Reject • Sentence.

6. Example 1 • A medical investigation claims that the average number of infections per week at a hospital in southwestern Pennsylvania is 16.3. A random sample of 10 weeks had a mean number of 17.7 infections. The sample standard deviation is 1.8. Is there enough evidence to reject the investigator’s claim at α = 0.05? • Step 1: State the hypotheses and identify the claim. H0:μ = 16.3 andH1:μ16.3 • Step 2: Find the critical value. • The critical values are 2.262 and –2.262 for α = 0.05 and d.f. = 9.

7. Step 3: Find the test value. • Step 4: Reject/Not • Step 5: Sentence • There is enough evidence to reject the claim that the average number of infections is 16.3.

8. An educator claims that the average salary of substitute teachers in school districts in Allegheny County, Pennsylvania, is less than \$60 per day. A random sample of eight school districts is selected, and the daily salaries (in dollars) are shown. Is there enough evidence to support the educator’s claim at α = 0.10? 60 56 60 55 70 55 60 55