Download 88.5)3(96.1 81 )27(96.1 = = = = nz error of margin

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A random sample of 81 credit sales in a department store showed an average sale of $68.00. From past data,
it is known that the standard deviation of the population is $27.00.
a. Determine the standard error of the mean.
27/sqrt(81) = 3
b. With a 0.95 probability, what can be said about the size of the margin of error?
zα σ
margin of error =
2
=
n
1.96(27)
= 1.96(3) = 5.88
81
c. What is the 95% confidence interval of the population mean?
(average - error margin , average + error margin) = (68 - 5.88, 68 + 5.88) = (62.12, 73.88)
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