In: Statistics and Probability
According to Thomson Financial, last year the majority of companies reporting profits had beaten estimates. A sample of 162 companies showed that 112 beat estimates, 29 matched estimates, and 21 fell short.
(a) | What is the point estimate of the proportion that fell short of estimates? If required, round your answer to four decimal places. |
pshort= | |
(b) | Determine the margin of error and provide a 95% confidence interval for the proportion that beat estimates. If required, round your answer to four decimal places. |
ME = | |
(c) | How large a sample is needed if the desired margin of error is 0.05? If required, round your answer to the next integer. |
Solution :
Given that,
n = 162
x = 21
(a)
Point estimate = sample proportion = = x / n = 21 / 162 = 0.130
(b)
x = 112
(a)
Point estimate = sample proportion = = x / n = 112 / 162 = 0.691
1 - = 0.309
Z/2 = Z 0.025 = 1.96
Margin of error = E = Z / 2 * (( * (1 - )) / n)
= 1.96 * (((0.691 * 0.309) / 162)
= 0.071
ME = 0.071
(c)
= 0.5
1 - = 1 - 0.5 = 0.5
margin of error = E = 0.05
Z/2 = Z0.025 = 1.96
sample size = n = (Z / 2 / E )2 * * (1 - )
= (1.96 / 0.05)2 * 0.5 * 0.5
= 385
sample size = n = 385