This problem relates to forming a confidence interval using a t-distribution. When using a t-distribution, the degrees of freedom (df) are calculated as:

$df = n - 1$

Where:

• $n$ is the sample size.

In the problem, 10 reels were used, so the sample size $n = 10$. Therefore, the degrees of freedom are:

$df = 10 - 1 = 9$

So, the correct answer is E) 9.

Would you like more details on this concept or have any other questions?

Related Questions:

1. How do you calculate confidence intervals for a population mean?
2. Why is the t-distribution used instead of the z-distribution in small samples?
3. What assumptions must be met for using the t-distribution in hypothesis testing?
4. How does the sample size affect the degrees of freedom in a t-test?
5. How does the shape of the t-distribution change as the degrees of freedom increase?

Tip:

Remember, as the sample size increases, the t-distribution approaches the normal distribution (z-distribution). This is why larger samples often use z-scores for confidence intervals.