Answer:
(a) The sample variance for the daily price change is 0.2501.
(b) The sample standard deviation for the daily price change is 0.5001.
(c) The 95% confidence interval estimates of the population variance is (0.1255, 0.7210).
Step-by-step explanation:
Let the random variable <em>X</em> denote the stock price changes for a sample of 12 companies on a day.
The data provided is:
<em>X</em> = {0.82
, 1.44
, -0.07
, 0.41
, 0.21
, 1.33
, 0.97
, 0.30
, 0.14
, 0.12
, 0.42
, 0.15}
(a)
The formula to compute the sample variance for the daily price change is:

The sample mean is computed using the formula:

Consider the Excel output attached below.
In Excel the formula to compute the sample mean and sample variance are:
=AVERAGE(A2:A13)
=VAR.S(A2:A13)
Thus, the sample variance for the daily price change is 0.2501.
(b)
The formula to compute the sample standard deviation for the daily price change is:

Consider the Excel output attached below.
In Excel the formula to compute the sample standard deviation is:
=STDEV.S(A2:A13)
Thus, the sample standard deviation for the daily price change is 0.5001.
(c)
The (1 - <em>α</em>)% confidence interval for population variance is:
![CI=[\frac{(n-1)s^{2}}{\chi^{2}_{\alpha/2} } \leq \sigma^{2}\leq \frac{(n-1)s^{2}}{\chi^{2}_{1-\alpha/2} } ]](https://tex.z-dn.net/?f=CI%3D%5B%5Cfrac%7B%28n-1%29s%5E%7B2%7D%7D%7B%5Cchi%5E%7B2%7D_%7B%5Calpha%2F2%7D%20%7D%20%5Cleq%20%5Csigma%5E%7B2%7D%5Cleq%20%5Cfrac%7B%28n-1%29s%5E%7B2%7D%7D%7B%5Cchi%5E%7B2%7D_%7B1-%5Calpha%2F2%7D%20%7D%20%5D)
Compute the critical value of Chi-square for <em>α</em> = 0.05 and (n - 1) = (12 - 1) = 11 degrees of freedom as follows:


*Use a Chi-square table.
Compute the 95% confidence interval estimates of the population variance as follows:
![CI=[\frac{(n-1)s^{2}}{\chi^{2}_{\alpha/2} } \leq \sigma^{2}\leq \frac{(n-1)s^{2}}{\chi^{2}_{1-\alpha/2} } ]](https://tex.z-dn.net/?f=CI%3D%5B%5Cfrac%7B%28n-1%29s%5E%7B2%7D%7D%7B%5Cchi%5E%7B2%7D_%7B%5Calpha%2F2%7D%20%7D%20%5Cleq%20%5Csigma%5E%7B2%7D%5Cleq%20%5Cfrac%7B%28n-1%29s%5E%7B2%7D%7D%7B%5Cchi%5E%7B2%7D_%7B1-%5Calpha%2F2%7D%20%7D%20%5D)
![=[\frac{(12-1)\times 0.2501}{21.920 } \leq \sigma^{2}\leq \frac{(12-1)\times 0.2501}{3.816} ]](https://tex.z-dn.net/?f=%3D%5B%5Cfrac%7B%2812-1%29%5Ctimes%200.2501%7D%7B21.920%20%7D%20%5Cleq%20%5Csigma%5E%7B2%7D%5Cleq%20%5Cfrac%7B%2812-1%29%5Ctimes%200.2501%7D%7B3.816%7D%20%5D)
![=[0.125506\leq \sigma^{2}\leq 0.720938]\\\approx [0.1255, 0.7210]](https://tex.z-dn.net/?f=%3D%5B0.125506%5Cleq%20%5Csigma%5E%7B2%7D%5Cleq%200.720938%5D%5C%5C%5Capprox%20%5B0.1255%2C%200.7210%5D)
Thus, the 95% confidence interval estimates of the population variance is (0.1255, 0.7210).