Answer:
[tex]\bar x = 260.1615[/tex]
[tex]\sigma = 70.69[/tex]
The confidence interval of standard deviation is: [tex]53.76[/tex] to [tex]103.25[/tex]
Step-by-step explanation:
Given
[tex]n =20[/tex]
See attachment for the formatted data
Solving (a): The mean
This is calculated as:
[tex]\bar x = \frac{\sum x}{n}[/tex]
So, we have:
[tex]\bar x = \frac{242.87 +212.00 +260.93 +284.08 +194.19 +139.16 +260.76 +436.72 +355.36 +.....+250.61}{20}[/tex]
[tex]\bar x = \frac{5203.23}{20}[/tex]
[tex]\bar x = 260.1615[/tex]
[tex]\bar x = 260.16[/tex]
Solving (b): The standard deviation
This is calculated as:
[tex]\sigma = \sqrt{\frac{\sum(x - \bar x)^2}{n-1}}[/tex]
[tex]\sigma = \sqrt{\frac{(242.87 - 260.1615)^2 +(212.00- 260.1615)^2+(260.93- 260.1615)^2+(284.08- 260.1615)^2+.....+(250.61- 260.1615)^2}{20 - 1}}[/tex][tex]\sigma = \sqrt{\frac{94938.80}{19}}[/tex]
[tex]\sigma = \sqrt{4996.78}[/tex]
[tex]\sigma = 70.69[/tex] --- approximated
Solving (c): 95% confidence interval of standard deviation
We have:
[tex]c =0.95[/tex]
So:
[tex]\alpha = 1 -c[/tex]
[tex]\alpha = 1 -0.95[/tex]
[tex]\alpha = 0.05[/tex]
Calculate the degree of freedom (df)
[tex]df = n -1[/tex]
[tex]df = 20 -1[/tex]
[tex]df = 19[/tex]
Determine the critical value at row [tex]df = 19[/tex] and columns [tex]\frac{\alpha}{2}[/tex] and [tex]1 -\frac{\alpha}{2}[/tex]
So, we have:
[tex]X^2_{0.025} = 32.852[/tex] ---- at [tex]\frac{\alpha}{2}[/tex]
[tex]X^2_{0.975} = 8.907[/tex] --- at [tex]1 -\frac{\alpha}{2}[/tex]
So, the confidence interval of the standard deviation is:
[tex]\sigma * \sqrt{\frac{n - 1}{X^2_{\alpha/2} }[/tex] to [tex]\sigma * \sqrt{\frac{n - 1}{X^2_{1 -\alpha/2} }[/tex]
[tex]70.69 * \sqrt{\frac{20 - 1}{32.852}[/tex] to [tex]70.69 * \sqrt{\frac{20 - 1}{8.907}[/tex]
[tex]70.69 * \sqrt{\frac{19}{32.852}[/tex] to [tex]70.69 * \sqrt{\frac{19}{8.907}[/tex]
[tex]53.76[/tex] to [tex]103.25[/tex]
