Duration of daylight in western Pennsylvania. What area of the United States has the least amount of daylight, on average? Having grown up in western Pennsylvania, co-author Sincich wonders if it is his hometown of Sharon, PA. Data on the number of minutes of daylight per day in Sharon, PA, for 12 randomly selected days (one each month) in 2009 were obtained from the Naval Oceanography Portal Web site ( aa.usno.navy.mil/USNO/astronomical-applications/ data-services ). The data are listed in the table and saved in the DAYLIGHT file. Descriptive statistics and a 95% confidence interval for the mean are produced in the SPSS printout on page 262.
a. Locate the confidence interval on the printout and give the value of the confidence coefficient.
b. Use the descriptive statistics on the printout to calculate the 95% confidence interval. Be sure your answer agrees with the interval shown on the printout.
c. Practically interpret the confidence interval.
d. Comment on the method of sampling. Do you think the sample is representative of the target population?
591 | 618 | 704 | 831 | 896 | 909 | 865 | 839 | 748 | 672 | 583 | 565 |
Based on Astronomical Applications Dept., U.S. Naval Observatory, Washington, DC.
SPSS Output for Exercise
|
|
| Statistic | Std. Error |
DAYLIGHT | Mean |
| 735.08 | 37.402 |
| 95% Confidence Interval for Mean | Lower Bound | 652.76 |
|
|
| Upper Bound | 817.40 |
|
| 5% Trimmed Mean |
| 734.87 |
|
| Median |
| 726.00 |
|
| Variance |
| 16786.992 |
|
| Std. Deviation |
| 129.565 |
|
| Minimum |
| 565 |
|
| Maximum |
| 909 |
|
| Range |
| 344 |
|
| Interquartile Range |
| 261 |
|
| Skewness |
| .016 | .637 |
| Kurtosis |
| -1.752 | 1.232 |
|
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|
|
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