A local bank needs information concerning the savings account balances of its customers. A random sample of 15 accounts was checked. The mean balance was $686.75 with a standard deviation of $256.20. Find a 98% confidence interval for the true mean. Assume that the account balances are normally distributed. Round to the nearest cent.

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**Determining a 98% Confidence Interval for Savings Account Balances**

A local bank is interested in gathering information about the savings account balances of its customers. To do this, they have selected a random sample of 15 savings accounts.

Here's the collected data:
- The mean (average) balance of these 15 accounts was $686.75.
- The standard deviation of the sample was $256.20.

**Objective:**
Calculate a 98% confidence interval for the true mean savings account balance, assuming that the account balances are normally distributed. The final answer should be rounded to the nearest cent.

**Method: Follow the PANIC acronym**

1. **P** (Parameter): Define the parameter of interest.
   - The parameter of interest is the true mean savings account balance of the bank’s customers.

2. **A** (Assumptions): Check the necessary assumptions.
   - The sample of 15 accounts is randomly selected.
   - The distribution of account balances is normal, as stated in the problem.

3. **N** (Name the Interval): Specify the name of the interval.
   - We will use a t-interval for the mean because the population standard deviation is unknown and the sample size is small (n < 30).

4. **I** (Interval Calculation): Calculate the interval.
   - The formula for a confidence interval for the mean using the t-distribution is:
     \[
     \bar{x} \pm t^* \left( \frac{s}{\sqrt{n}} \right)
     \]
     Where:
     - \(\bar{x}\) is the sample mean ($686.75)
     - \(s\) is the sample standard deviation ($256.20)
     - \(n\) is the sample size (15)
     - \(t^*\) is the t-value for 98% confidence and 14 degrees of freedom (since degrees of freedom = n - 1 = 14).

     Using a t-table or calculator to find \(t^*\) for 14 degrees of freedom at 98% confidence, we get approximately 2.977.

     Plugging the values into the formula:
     \[
     686.75 \pm 2.977 \left( \frac{256.20}{\sqrt{15}} \right)
     \]

     \[
     686.75 \pm 2.977 \left( \frac
Transcribed Image Text:**Determining a 98% Confidence Interval for Savings Account Balances** A local bank is interested in gathering information about the savings account balances of its customers. To do this, they have selected a random sample of 15 savings accounts. Here's the collected data: - The mean (average) balance of these 15 accounts was $686.75. - The standard deviation of the sample was $256.20. **Objective:** Calculate a 98% confidence interval for the true mean savings account balance, assuming that the account balances are normally distributed. The final answer should be rounded to the nearest cent. **Method: Follow the PANIC acronym** 1. **P** (Parameter): Define the parameter of interest. - The parameter of interest is the true mean savings account balance of the bank’s customers. 2. **A** (Assumptions): Check the necessary assumptions. - The sample of 15 accounts is randomly selected. - The distribution of account balances is normal, as stated in the problem. 3. **N** (Name the Interval): Specify the name of the interval. - We will use a t-interval for the mean because the population standard deviation is unknown and the sample size is small (n < 30). 4. **I** (Interval Calculation): Calculate the interval. - The formula for a confidence interval for the mean using the t-distribution is: \[ \bar{x} \pm t^* \left( \frac{s}{\sqrt{n}} \right) \] Where: - \(\bar{x}\) is the sample mean ($686.75) - \(s\) is the sample standard deviation ($256.20) - \(n\) is the sample size (15) - \(t^*\) is the t-value for 98% confidence and 14 degrees of freedom (since degrees of freedom = n - 1 = 14). Using a t-table or calculator to find \(t^*\) for 14 degrees of freedom at 98% confidence, we get approximately 2.977. Plugging the values into the formula: \[ 686.75 \pm 2.977 \left( \frac{256.20}{\sqrt{15}} \right) \] \[ 686.75 \pm 2.977 \left( \frac
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