Twelve different video games showing substance use were observed, and the durations of gameplay (in seconds) are listed below. The design of the study justifies the times of gameplay. Assume that this sample was obtained from a population with a normal distribution.

\begin{tabular}{llllll}
4,546 & 4,797 & 4,126 & 4,589 & 4,336 & 4,092 \\
4,665 & 4,777 & 3,848 & 4,060 & 4,572 & 4,277
\end{tabular}

The confidence interval estimate is [tex]$\square \ \text{sec} \ \textless \ \sigma \ \textless \ \square \ \text{sec}$[/tex].
(Round to one decimal place as needed)



Answer :

To construct a confidence interval for the population standard deviation [tex]\(\sigma\)[/tex] based on the sample data, we need a few key statistics and steps. Here, I'll break down the entire process for you.

### Step 1: Understanding the Data
Given the data set:
[tex]\[ 4546, 4797, 4126, 4589, 4336, 4092, 4665, 4777, 3848, 4060, 4572, 4277 \][/tex]
The sample size ([tex]\(n\)[/tex]) is 12.

### Step 2: Calculating the Sample Variance
The sample variance is used as an estimator for the variance of the population. The formula for sample variance [tex]\(s^2\)[/tex] is:
[tex]\[ s^2 = \frac{1}{n-1} \sum_{i=1}^n (x_i - \bar{x})^2 \][/tex]
where [tex]\( \bar{x} \)[/tex] is the sample mean.

Here, the sample variance is given as:
[tex]\[ s^2 = 96813.72 \][/tex]

### Step 3: Degrees of Freedom
The degrees of freedom ([tex]\( \text{df} \)[/tex]) for calculating the variance and the subsequent chi-square distribution is:
[tex]\[ \text{df} = n - 1 = 12 - 1 = 11 \][/tex]

### Step 4: Confidence Level
We are constructing a [tex]\(95\%\)[/tex] confidence interval. The confidence level [tex]\( (1 - \alpha) \)[/tex] is [tex]\(0.95\)[/tex], hence:
[tex]\[ \alpha = 1 - 0.95 = 0.05 \][/tex]

### Step 5: Chi-Square Critical Values
We use the chi-square distribution to get the critical values for our confidence interval. They are:
- Lower critical value ([tex]\( \chi^2_{\frac{\alpha}{2}, \text{df}} \)[/tex]): [tex]\(3.816\)[/tex]
- Upper critical value ([tex]\( \chi^2_{1-\frac{\alpha}{2}, \text{df}} \)[/tex]): [tex]\(21.920\)[/tex]

### Step 6: Calculating the Confidence Interval for Variance
The formula for the confidence interval for the variance [tex]\(\sigma^2\)[/tex] is:
[tex]\[ \left( \frac{(n-1)s^2}{\chi^2_{1-\frac{\alpha}{2}, \text{df}}}, \frac{(n-1)s^2}{\chi^2_{\frac{\alpha}{2}, \text{df}}} \right) \][/tex]

Plugging in:
[tex]\[ \left( \frac{(11) \cdot 96813.72}{21.920}, \frac{(11) \cdot 96813.72}{3.816} \right) \][/tex]
[tex]\[ = (48583.42, 279093.60) \][/tex]

### Step 7: Calculating the Confidence Interval for Standard Deviation
To find the confidence interval for the standard deviation [tex]\(\sigma\)[/tex], we take the square root of the interval bounds for the variance:
[tex]\[ \left( \sqrt{48583.42}, \sqrt{279093.60} \right) \][/tex]
[tex]\[ \approx (220.4 \text{ seconds}, 528.3 \text{ seconds}) \][/tex]

### Step 8: Final Confidence Interval
Therefore, the [tex]\(95\% \)[/tex] confidence interval for the population standard deviation [tex]\(\sigma\)[/tex] based on the sample data is:
[tex]\[ 220.4 \text{ seconds} < \sigma < 528.3 \text{ seconds} \][/tex]

This is the bounded interval that estimates the standard deviation of the duration of times of gameplay featuring substance use in this study.

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