Question

A professor wishes to determine if his grading is consistent, so he reviews the grades of a sample of students and determines that 7% earned an A. In each of his current classes with 25 enrolled, between what two class percentages of students earning A's should he to occur 90% of the time?

And how would this be put into Excel?

Answer 1

Here = 0.07 and = 1 - 0.07 = 0.93

Here n* and n (1-) are greater than 10, so we use the Z critical values(2 tail) at = 0.10 is 1.645

The Confidence Interval is given by:

Where the ME =

The Lower Limit = 0.07 - 0.084 = -0.014

The Upper Limit = 0.07 + 0.084 = 0.154

Therefore, a 90% confidence interval for the population proportion is (-0.014 , 0.154).

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The Excel output is as below

Single Proportion		x	n
Alpha	0.1	1.175	25
Round phat	4
critical	1.6450	(NORMSINV((1-0.1),2)
phat	0.0700
1-phat	0.9300
n	25
b*c/n	0.002604
SE = SQRT(d)	0.051029403
ME = Zc * SE	0.08394
	4 DP	3 DP
Lower	-0.0139	-0.014
Upper	0.1539	0.154