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Z Critical Value:
A critical value of z is sometimes written as za, where the alpha level, a, is the area in the tail. For example, z.10=1.28. When are Critical values of z used? A critical value of z (Z-score) is used when the sampling distribution is normal, or close to normal.

Critical Values of z

α=tail area	central area = 1 – 2α	zα
0.10	0.80	z.10=1.28
0.05	0.90	z.05=1.645
0.025	0.95	z.025=1.96
0.01	0.98	z.01=2.33
0.005	0.99	z.005=2.58

This calculator calculates the critical value using tail area values.