Whenever you come across the term **z _{α/2}** in statistics, it is simply referring to the

**z critical value**from the z table that corresponds to α/2.

This tutorial explains the following:

- How to find z
_{α/2}using a z table. - How to find z
_{α/2}using a calculator. - The most common values for z
_{α/2}.

Let’s jump in!

**How to find z**_{α/2} using a z table

_{α/2}using a z table

Suppose we want to find z_{α/2} for some test that is using a 90% confidence level.

In this case, α would be 1 – 0.9 = **0.1**. Thus, α/2 = 0.1/2 = **0.05**.

To find the corresponding z critical value, we would simply look for **0.05** in a z table:

Notice that the exact value of **0.05 **doesn’t appear in the table, but it would be directly between the values **.0505 **and **.0495**. The corresponding z critical values on the outside of the table are **-1.64** and **-1.65**.

By splitting the difference, we see that the z critical value would be **-1.645**. And typically when we use z_{α/2} we take the absolute value. Thus, z_{.01/2} = **1.645**.

**How to find z**_{α/2} using a calculator

_{α/2}using a calculator

We can also use a Critical Z Value Calculator to find z_{α/2} for some test.

For example, for some test that is using a 90% confidence level we can simply enter **0.1 **as the significance level and the calculator will automatically return the value of **1.645 **as the corresponding critical z value:

**Common Values for z**_{α/2}

_{α/2}

The following table displays the most common critical values for different values of α:

The way to interpret this table is as follows:

- For a test using a 90% confidence level (e.g. α = 0.1), the z critical value is
**1.645**. - For a test using a 95% confidence level (e.g. α = 0.05), the z critical value is
**1.96**. - For a test using a 99% confidence level (e.g. α = 0.01), the z critical value is
**5.576**.

And so on.

**Additional Resources**

How to use the Z Table (With Examples)

How to Find the Z Critical Value on a TI-84 Calculator

The 99% critical value typed at the end is different from the value in the table.

Hello I am trying to reference this page would you be able to share your last name to put at the author for a stats module at university

There is a typo in your values:

The way to interpret this table is as follows:

For a test using a 90% confidence level (e.g. α = 0.1), the z critical value is 1.645.

For a test using a 95% confidence level (e.g. α = 0.05), the z critical value is 1.96.

For a test using a 99% confidence level (e.g. α = 0.01), the z critical value is 5.576.

where a 99% confidence levels critical value should be 2.576

I believe you have a typo for the z critical value for 99% confidence level in the “The way to interpret this table” section. It says 5.576 instead of 2.576.

None of the books and sites I visited explained this as clearly as you did. THANK YOU.