A foundry run of steel is sampled to ensure the density is m

A foundry run of steel is sampled to ensure the density is meeting customers expectations. 256 samples are taken, they have an average density of 7950 kg/m³ , and standard deviation of 520 kg/m³ .

Find: Determine the 90% confidence interval on that average, and explain what that range represents.

Solution

Sample mean = 7950

s = sample std dev=520

n = sample size = 256

90% z value = 1.64

std error = 520/rt256 = 32.5

Hence lower bound for 90% conf interval = 7950-1.64(32.5) = 7896.7

Upper bound = 8003.33