The Z critical value for a 95% confidence interval is:

• 1.96 for a two-tailed test;
• 1.64 for a right-tailed test; and
• -1.64 for a left-tailed test.

To find a Z critical value for a given confidence level α:

1. Check if you perform a one- or two-tailed test.
2. For a one-tailed test:
   • Left-tailed: critical value is the α-th quantile of the standard normal distribution N(0,1).
   • Right-tailed: critical value is the (1-α)-th quantile.
3. Two-tailed test: critical value equals ±(1-α/2)-th quantile of N(0,1).
4. No quantile tables? Use CDF tables! (The quantile function is the inverse of the CDF.)
5. Verify your answer with an online critical value calculator.

In theory, no. In practice, very often, yes. The t-Student distribution is similar to the standard normal distribution, but it is not the same. However, if the number of degrees of freedom (which is, roughly speaking, the size of your sample) is large enough (>30), then the two distributions are practically indistinguishable, and so the t critical value has practically the same value as the Z critical value.