Confidence Interval for a Mean

Confidence Interval for a Mean

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Confidence Interval for a Mean

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1. Confidence Interval for a Mean when you have a “small” sample...

2. As long as you have a “large” sample…. A confidence interval for a population mean is: where the average, standard deviation, and n depend on the sample, and Z depends on the confidence level.

3. Random sample of 59 students spent an average of \$273.20 on Spring 1998 textbooks. Sample standard deviation was \$94.40. Example We can be 95% confident that the average amount spent by all students was between \$249.11 and \$297.29.

4. What happens if you can only take a “small” sample? • Random sample of 15 students slept an average of 6.4 hours last night with standard deviation of 1 hour. • What is the average amount all students slept last night?

5. If you have a “small” sample... Replace the Z value with a t value to get: where “t” comes from Student’s t distribution, and depends on the sample size through the degrees of freedom “n-1”.

6. Student’s t distribution versus Normal Z distribution

7. T distribution • Shaped like standard normal distribution (symmetric around 0, bell-shaped). • But, t depends on the degrees of freedom “n-1”. • And, more likely to get extreme t values than extreme Z values.

8. Graphical Comparison of T and Z Multipliers

9. Tabular Comparison of T and Z Multipliers For small samples, T value is larger than Z value. So,T interval is made to be longer than Z interval.

10. Back to our example! Sample of 15 students slept an average of 6.4 hours last night with standard deviation of 1 hour. Need t with n-1 = 15-1 = 14 d.f. For 95% confidence, t14 = 2.145

11. That is... We can be 95% confident that average amount slept last night by all students is between 5.85 and 6.95 hours. Hmmm! Adults need 8 hours of sleep each night. Logical conclusion: On average, students need more sleep. (Just don’t get it in this class!)

12. T-Interval for Mean in Minitab T Confidence Intervals Variable N Mean StDev SE Mean 95.0 % CI Comb 89 2.011 1.563 0.166 (1.682, 2.340) We can be 95% confident that the average number of times a “Stat-250-like” student combs his/her is between 1.7 and 2.3 times a day.

13. T- interval in Minitab • Select Stat. • Select Basic Statistics. • Select 1-Sample t… • Select desired variable. • Specify desired confidence level. • Say OK.

14. What happens as sample gets larger?

15. What happens to CI as sample gets larger? For large samples: Z and t values become almost identical, so CIs will be almost identical.

16. Example Random sample of 64 students spent an average of 3.8 hours on homework last night with a sample standard deviation of 3.1 hours. Z Confidence Intervals The assumed sigma = 3.10 Variable N Mean StDev 95.0 % CI Homework 64 3.797 3.100 (3.037, 4.556) T Confidence Intervals Variable N Mean StDev 95.0 % CI Homework 64 3.797 3.100 (3.022, 4.571)

17. One not-so-small problem! • It is only OK to use the t interval for small samples if your original measurements are normally distributed. • We’ll learn how to check for normality.

18. Strategy • If you have a large sample of, say, 30 or more measurements, then don’t worry about normality, and calculate a t-interval. • If you have a small sample and your data are normally distributed, then calculate a t-interval. • If you have a small sample and your data are not normally distributed, then stay tuned.