Find critical values for one and two-tailed tests at a chosen significance level.
A two-tailed test splits the significance level between both tails, so its critical value is larger than a one-tailed test at the same alpha. At 5 per cent these are 1.96 and 1.645 respectively. Choosing one-tailed after seeing the data inflates the false positive rate, which is why the direction must be specified in advance.
Critical Value
The critical value is the z with tail area α (one-tailed) or α/2 (two-tailed)
The critical value is the z with tail area α (one-tailed) or α/2 (two-tailed) A two-tailed test splits the significance level between both tails, so its critical value is larger than a one-tailed test at the same alpha. At 5 per cent these are 1.96 and 1.645 respectively.
Choosing one-tailed after seeing the data inflates the false positive rate, which is why the direction must be specified in advance.
This calculator takes 3 inputs: Significance level, Test type, Your test statistic. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.