Posted by Lorie on Thursday, October 13, 2011 at 1:50pm.
An auditor wants to estimate what proportion of a bank’s commercial loan files are incomplete. The auditor wants to be within 10% of the true proportion when using a 95% confidence level. How many files must the auditor sample? No estimate of the proportion is available, so use 0.5 for the population proportion.

Statistics  MathGuru, Friday, October 14, 2011 at 6:00pm
Here is one formula you might use for this problem:
n = [(zvalue)^2 * p * q]/E^2
With your data:
n = [(1.96)^2 * .5 * .5]/.10^2
I'll let you finish the calculation (round to the next highest whole number).
Note: n = sample size needed; .5 for best estimate of the population proportion; .5 for q, which is (1  p). E = maximum error, which is 10% or .10 from the problem. Zvalue is found using a ztable (for 95%, the value is 1.96).
I hope this helps.
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