Because we can compute the population standard deviation, we use the z-statistic. A 90% confidence level is constructed by taking the population mean and adding and subtracting the product of the z-statistic reliability (z_α/2) factor times the known standard deviation of the population divided by the square root of the sample size (note that the population variance is given and its positive square root is the standard deviation of the population): x ± z_α/2 × ( σ / n^1/2) = 50 ± 1.645 × (900^1/2 / 100^1/2) = 50 ± 1.645 × (30 / 10) = 50 ± 1.645 × (3). This is interpreted to mean that we are 90% confident that the above interval contains the true mean starting salaries of CFA charterholders.