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Answer:
1. The 95% confidence interval would be given by (24.8190;27.8010)
2.
3.
Since our calculated value it's higher than the critical value we have enough evidence at 5% of significance to reject th null hypothesis and conclude that the true mean is higher than 25 cents per mile.
4.
Since our calculated value is less than the critical value we don't hav enough evidence to reject the null hypothesis at the significance level provided.
Step-by-step explanation:
Previous concepts
represent the sample mean for the sample
population mean (variable of interest)
s=2.8 represent the sample standard deviation
n=16 represent the sample size
Part 1
The confidence interval for the mean is given by the following formula:
(1)
In order to calculate the critical value we need to find first the degrees of freedom, given by:
Since the Confidence is 0.95 or 95%, the value of and
, and we can use excel, a calculator or a tabel to find the critical value. The excel command would be: "=-T.INV(0.025,15)".And we see that
Now we have everything in order to replace into formula (1):
So on this case the 95% confidence interval would be given by (24.8190;27.8010)
Part 2
The confidence interval for the population variance is given by the following formula:
The next step would be calculate the critical values. First we need to calculate the degrees of freedom given by:
Since the Confidence is 0.90 or 90%, the value of and
, and we can use excel, a calculator or a table to find the critical values.
The excel commands would be: "=CHISQ.INV(0.05,15)" "=CHISQ.INV(0.95,15)". so for this case the critical values are:
And replacing into the formula for the interval we got:
Part 3
We need to conduct a hypothesis in order to determine if actual mean operating cost is at most 25 cents per mile , the system of hypothesis would be:
Null hypothesis:
Alternative hypothesis:
We don't know the population deviation, so for this case is better apply a t test to compare the actual mean to the reference value, and the statistic is given by:
(1)
Calculate the statistic
We can replace in formula (1) the info given like this:
Critical value
On this case we need a critical value on th t distribution with 15 degrees of freedom that accumulates 0.05 of th area on the right and 0.95 of the area on the left. We can calculate this value with the following excel code:"=T.INV(0.95,15)" and we got
Conclusion
Since our calculated valu it's higher than the critical value we have enough evidence at 5% of significance to reject th null hypothesis and conclude that the true mean is higher than 25 cents per mile.
Part 4
State the null and alternative hypothesis
On this case we want to check if the population standard deviation is more than 2.3, so the system of hypothesis are:
H0:
H1:
In order to check the hypothesis we need to calculate the statistic given by the following formula:
This statistic have a Chi Square distribution distribution with n-1 degrees of freedom.
What is the value of your test statistic?
Now we have everything to replace into the formula for the statistic and we got:
What is the critical value for the test statistic at an α = 0.05 significance level?
Since is a right tailed test the critical zone it's on the right tail of the distribution. On this case we need a quantile on the chi square distribution with 15 degrees of freedom that accumulates 0.05 of the area on the right tail and 0.95 on the left tail.
We can calculate the critical value in excel with the following code: "=CHISQ.INV(0.95,15)". And our critical value would be
Since our calculated value is less than the critical value we FAIL to reject the null hypothesis.