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November 23rd, 2013, 06:25 PM   #1
Joined: Oct 2013

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sample size

How many concrete cubes should a construction site Materials Engineer take in each
sample (batch) if the Contract Specifications require that he should be 90% confident that
his estimate of the mean concrete strength does not deviate from the true mean strength of
that day’s concrete by 3 N/mm2. From past experience, it has been found that the strengths
of concrete produced are normally distributed with a variance of 12.25 N/mm2.

For a normally distributed population, how many samples should we take in order to
ensure that there is a 98% chance of obtaining a sample mean falling between 105 and
115? The population mean and variance are 110 and 86 respectively.
Engr Faisal hbaib is offline  

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