# Confidence Interval Calculator

## Confidence Interval for the population means when the standard deviation is known.

Confidence level, usually written as $(1-\alpha)$100%, on the interval estimate of a population parameter is the probability that the interval estimate will contain the true population parameter.  When $/apha=0.05,95% is the confidence level and 0.95 is the probability that the interval estimate will have the population parameter.$

Value of z for different values of $\alpha$
Confidence Interval $\alpha$ z
80% 0.200 1.282
85% 0.150 1.440
90% 0.100 1.645
95% 0.050 1.960
99% 0.010 2.576
99.5% 0.005 2.807
99.9% 0.009 3.291

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