You plan to estimate the mean incubation period (in days) for sea birds. How many incubation periods of sea birds must you sample if you want to be 95% confident that the sample mean is within 3 days of the true population mean? You can assume that the standard deviation of incubation periods is 6 days.

Given that margin of error (E) = 3

Standard deviation = 6

z critical = 1.96

Sample size n = (Ez)2=(31.966)2=15.366416

Why do we use that the approx is 16 and not 15