In: Statistics and Probability
A bank wonders whether omitting the annual credit card fee for
customers who charge at leats $2500 in a year will increase the
amount charged on its credit cards. The bank makes this offer to an
SRS of 200 of its credit card customers. It then compares how much
these customers charge this year with the amount that they charged
last year. The mean increase in the sample is $346, and the
standard deviation is $112. Give a 99% confidence interval for the
mean amount charges would have incremented if this benefit had been
extended to all such customers.
Z* for 99% confidence is 2.58.
Solution :
Given that,
Point estimate = sample mean =
= 346
Population standard deviation =
= 112
Sample size = n = 200
At 99% confidence level
= 1 - 99%
= 1 - 0.99 =0.01
/2
= 0.005
Z/2
= Z0.005 = 2.58
Margin of error = E = Z/2
* (
/n)
= 2.58 * (112 / 200
)
= 20.43
At 99% confidence interval estimate of the population mean is,
± E
346 ± 20.43
( $ 325.57, $ 366.43 )