Z-Score
An obvious outlier among ten values scores 2.8315, which is 99.5% of the maximum that sample size allows.
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Z-Test
Estimate minus null value, over a known standard error, against the normal curve. Exact for normal data with known sigma, else approximate; mostly proportions.
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Normal Distribution
Most forecast intervals assume normality, and the assumption is least reliable exactly in the tails — which is where risk questions live.