Answer:
a)
So with the p value obtained and using the significance level given
we have
so we can conclude that we have enough evidence to reject the null hypothesis.
b) ![(91-85) -2.79 *2.490 \sqrt{\frac{1}{12} +\frac{1}{15}} =3.309](https://tex.z-dn.net/?f=%20%2891-85%29%20-2.79%20%2A2.490%20%5Csqrt%7B%5Cfrac%7B1%7D%7B12%7D%20%2B%5Cfrac%7B1%7D%7B15%7D%7D%20%3D3.309)
![(91-85) +2.79 *2.490 \sqrt{\frac{1}{12} +\frac{1}{15}} =8.691](https://tex.z-dn.net/?f=%20%2891-85%29%20%2B2.79%20%2A2.490%20%5Csqrt%7B%5Cfrac%7B1%7D%7B12%7D%20%2B%5Cfrac%7B1%7D%7B15%7D%7D%20%3D8.691)
Step-by-step explanation:
Notation and hypothesis
When we have two independent samples from two normal distributions with equal variances we are assuming that
And the statistic is given by this formula:
Where t follows a t distribution with
degrees of freedom and the pooled variance
is given by this formula:
This last one is an unbiased estimator of the common variance
Part a
The system of hypothesis on this case are:
Null hypothesis:
Alternative hypothesis:
Or equivalently:
Null hypothesis:
Alternative hypothesis:
Our notation on this case :
represent the sample size for group 1
represent the sample size for group 2
represent the sample mean for the group 1
represent the sample mean for the group 2
represent the sample standard deviation for group 1
represent the sample standard deviation for group 2
First we can begin finding the pooled variance:
And the deviation would be just the square root of the variance:
Calculate the statistic
And now we can calculate the statistic:
Now we can calculate the degrees of freedom given by:
Calculate the p value
And now we can calculate the p value using the altenative hypothesis:
Conclusion
So with the p value obtained and using the significance level given
we have
so we can conclude that we have enough evidence to reject the null hypothesis.
Part b
For this case the confidence interval is given by:
![(\bar X_1 -\bar X_2) \pm t_{\alpha/2} S_p \sqrt{\frac{1}{n_1} +\frac{1}{n_2}}](https://tex.z-dn.net/?f=%20%28%5Cbar%20X_1%20-%5Cbar%20X_2%29%20%5Cpm%20t_%7B%5Calpha%2F2%7D%20S_p%20%5Csqrt%7B%5Cfrac%7B1%7D%7Bn_1%7D%20%2B%5Cfrac%7B1%7D%7Bn_2%7D%7D)
For the 99% of confidence we have
and
and the critical value with 25 degrees of freedom on the t distribution is ![t_{\alpha/2}= 2.79](https://tex.z-dn.net/?f=%20t_%7B%5Calpha%2F2%7D%3D%202.79)
And replacing we got:
![(91-85) -2.79 *2.490 \sqrt{\frac{1}{12} +\frac{1}{15}} =3.309](https://tex.z-dn.net/?f=%20%2891-85%29%20-2.79%20%2A2.490%20%5Csqrt%7B%5Cfrac%7B1%7D%7B12%7D%20%2B%5Cfrac%7B1%7D%7B15%7D%7D%20%3D3.309)
![(91-85) +2.79 *2.490 \sqrt{\frac{1}{12} +\frac{1}{15}} =8.691](https://tex.z-dn.net/?f=%20%2891-85%29%20%2B2.79%20%2A2.490%20%5Csqrt%7B%5Cfrac%7B1%7D%7B12%7D%20%2B%5Cfrac%7B1%7D%7B15%7D%7D%20%3D8.691)