Question

I am having an issue with my school's math program but first want to ensure it is the program and not my process.

Question-

You want to obtain a sample to estimate a population proportion. At this point in time, you have no reasonable preliminary estimation for the population proportion. You would like to be 99% confident that you estimate is within 3.5% of the true population proportion. How large of a sample size is required?

(1-0.99)/2 = 0.005 invNorm of 0.005 (or more accurately 0.995) is 2.5758 so the using the formula 0.25(2.5758/0.035)^2 = 1354.03 or 1354 rounded to nearest whole number. Per the program this is incorrect and the answer is 1355.

There is no possible way to round up to 1355 so something is off.

I started taking this to mean that any value over the whole number should be rounded up to the next whole number (I dont agree with this mathematically but ok) and then that failed on another question where my answer was 4147.36 so I rounded to 4148 and the correct answer was 4147. Please help me figure this out...thank you

Answer 1

Refer Standard normal table/Z-table or use excel function "=NORM.S.INV((1-0.005))" to find the Z-value.