Question 1187095
Here's how to estimate the mean difference in traffic count with a 90% confidence level:

1. **Calculate the Differences:**  Find the difference between the traffic count on the 6th and the 13th for each pair of dates.  (6th - 13th)

   *   1990, July: 139246 - 138548 = 698
   *   1990, July: 134012 - 132908 = 1104
   *   1991, September: 137055 - 136018 = 1037
   *   1991, September: 133732 - 131843 = 1889
   *   1991, December: 123552 - 121641 = 1911
   *   1991, December: 121139 - 118723 = 2416
   *   1992, March: 128293 - 125532 = 2761
   *   1992, March: 124631 - 120249 = 4382
   *   1992, November: 124609 - 122770 = 1839
   *   1992, November: 117584 - 117263 = 321

2. **Calculate the Sample Mean Difference (d̄):** Sum the differences and divide by the number of pairs (n = 10).

   d̄ = (698 + 1104 + 1037 + 1889 + 1911 + 2416 + 2761 + 4382 + 1839 + 321) / 10
   d̄ = 18358 / 10
   d̄ = 1835.8

3. **Calculate the Sample Standard Deviation of the Differences (sd):**

   First, find the squared difference from the mean for each difference and sum them.

   Then, divide the sum by (n-1) and take the square root.

   sd ≈ 1220.17

4. **Find the t-score:** For a 90% confidence level and 9 degrees of freedom (n-1 = 10-1 = 9), the t-score (from a t-table or calculator) is approximately 1.833.

5. **Calculate the Margin of Error:**

   Margin of Error = t-score * (sd / √n)
   Margin of Error = 1.833 * (1220.17 / √10)
   Margin of Error ≈ 706.85

6. **Calculate the Confidence Interval:**

   Confidence Interval = d̄ ± Margin of Error
   Confidence Interval = 1835.8 ± 706.85
   Confidence Interval ≈ (1128.95, 2542.65)

**Conclusion:**

We are 90% confident that the true mean difference in traffic count between the 6th and the 13th is between approximately 1128.95 and 2542.65.  Since the interval is positive, this suggests that traffic counts tend to be lower on Friday the 13th.