Returns a confidence interval.


CONFIDENCE(α, sd, size)

sd (> 0) is the (known) standard deviation of a normal distribution.
size is the size of a sample from that distribution.
α is the significance level (0 < α < 1), which determines the desired confidence level = (1 - α)*100%. Thus for example α = 0.05 gives a 95% confidence level.
CONFIDENCE returns a value that when added and subtracted from the sample mean gives an interval within which the population mean is expected to lie with the specified confidence level.
CONFIDENCE calculates NORMINV(1 - α/2; 0; 1) * sd / √(size)


returns approximately 0.438. With a 95% confidence level, the mean of the entire population lies within ±0.438 of the sample mean.

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