# Answer to Question #17338 in Statistics and Probability for Dion

Question #17338

If the standard deviation of a national accounting examination is 900, how large a sample is needed to estimate the true mean score within 5 points with 99% confidence?

Expert's answer

Variance = 900

Standard deviation = 30

z(alpha) = 2.575

error = 5

n = ( z(alpha) * SD /error)² = ( 2.575 * 30 / 5 )² = 238.7025

sample of 239 is needed.

Standard deviation = 30

z(alpha) = 2.575

error = 5

n = ( z(alpha) * SD /error)² = ( 2.575 * 30 / 5 )² = 238.7025

sample of 239 is needed.

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