Statistics How To

Critical values are used in tests of statistical significance. The alpha level is the maximum probability where you reject the null hypothesis if the null hypothesis is true (in other words, you’re controlling the possibility of Type 1 errors to a certain level). Let’s say you have an alpha level of .05 for this example.

Step 1: Subtract alpha from 1.

1 – .05 = .95

Step 2 :Divide Step 1 by 2 (because we are looking for a two-tailed test)

.95 / 2 = .475

Step 3: Look at your z-table and locate the alpha level in the middle section of the z-table. The fastest way to do this is to use an online z-table and use the find function of your browser (usually CTRL+F). In this example we’re going to look for .475, so go ahead and press CTRL+F, then type in .475.

Step 4:. In this example, you should have found the number .4750. Look to the far left or the row, you’ll see the number 1.9 and look to the top of the column, you’ll see .06. Add them together to get 1.96. That’s the critical value!

Tip: The critical value appears twice in the z table because you’re looking for both a left hand and a right hand tail, so don’t forget to add plus or minus! In our example you’d get ±1.96.

Related posts:

1. How to Find a Critical Value for a Left-Tailed Test
2. How to find a critical value in statistics (two-tailed test)
3. How to Find a Critical Value for a Right-Tailed Test
4. How to Find a Critical Value on a Ti-89 Titanium (Left-Tailed test)
5. How to Decide if a Hypothesis Test is a One Tailed Test or a Two Tailed Test