Question

Suppose that a random sample of 12 adults has a mean score of 67 on a standardized personality test, with a standard deviation of 5. (A higher score indicates a more personable participant.) If we assume that scores on this test are normally distributed, find a 90% confidence interval for the mean score of all takers of this test. Then complete the table below. Carry your intermediate computations to at least three decimal places. Round your answers to one decimal place. (If necessary, consult a list of formulas.) What is the lower limit of the confidence interval? What is the upper limit of the confidence interval?

Answer 1

`67-1.8(5)/(√(12))=64.4`

`67+1.8(5)/(√(12))=69.6`

So on this case the 90% confidence interval would be given by (64.4;69.6)

Explanation:

Previous concepts

A confidence interval is "a range of values that’s likely to include a population value with a certain degree of confidence. It is often expressed a % whereby a population means lies between an upper and lower interval".

The margin of error is the range of values below and above the sample statistic in a confidence interval.

Normal distribution, is a "probability distribution that is symmetric about the mean, showing that data near the mean are more frequent in occurrence than data far from the mean".

`\bar X=67` represent the sample mean for the sample

`\mu` population mean (variable of interest)

s=5 represent the sample standard deviation

n=1 represent the sample size

Solution to the problem

The confidence interval for the mean is given by the following formula:

`\bar X \pm t_(\alpha/2)(s)/(√(n))` (1)

In order to calculate the critical value
`t_(\alpha/2)` we need to find first the degrees of freedom, given by:

`df=n-1=12-1=11`

Since the Confidence is 0.90 or 90%, the value of
`\alpha=0.1` and
`\alpha/2 =0.05`, and we can use excel, a calculator or a tabel to find the critical value. The excel command would be: "=-T.INV(0.05,11)".And we see that
`t_(\alpha/2)=1.80`

Now we have everything in order to replace into formula (1):

`67-1.8(5)/(√(12))=64.4`

`67+1.8(5)/(√(12))=69.6`

So on this case the 90% confidence interval would be given by (64.4;69.6)