Syntax: CONFIDENCE(Alpha, SDev, N)
Alpha = significance level
SDev = standard deviation
N = sample size
CONFIDENCE returns the two-sided Alpha - 1 confidence interval for the mean of a normally distributed population with a standard deviation of SDev given a sample size of N. Alpha must be greater than zero and less than one. SDev must be greater than zero. N must be an integer value greater than zero. If N is not an integer, it is truncated such that the fractional portion is removed; i.e., an N value of 10.9 will be treated as 10.
Suppose A1..A50 contains 50 samples of a random variable with a computed average of 14.73 and standard deviation of 1.19; i.e., AVERAGE(A1..A50) = 14.73 and STDEV(A1..A50) = 1.19. Then the 95% confidence interval for the mean of the population from which the samples were taken is given by:
CONFIDENCE(0.05, 1.19, 50) = 0.32984511
Therefore, the 95% confidence interval is 14.73 plus or minus 0.32984511, which is 14.400155 to 15.059845.
Excel function: CONFIDENCE