Main > Critical Values & Hypothesis Testing > Find a critical value

Critical values are used in hypothesis testing. The alpha level is the maximum probability where you **reject the null hypothesis** if the null hypothesis is true. The technique for looking up a critical value is very straightforward as long as you know if you have a left tailed test or right tailed test (or potentially, both).

**Click to skip to the steps for the type of test:**

Find a critical value for a confidence interval

Common confidence intervals and their critical values

Find a Critical Value: Two-Tailed Test

Find a Critical Value: Right-Tailed Test

Find a Critical Value: Left-Tailed Test

## Find a critical value for a confidence interval

**Sample question:**Find a critical value for a 90% confidence level.

Step 1: Subtract the confidence level from 100% to find the α level: 100% – 90% = 10%.

Step 2: Convert Step 1 to a decimal: 10% = 0.10.

Step 3: Divide Step 2 by 2 (this is called “α/2″).

0.10 = 0.05. This is the area in each tail.

Step 4: Subtract Step 3 from 1 (because we want the area in the middle, not the area in the tail):

1 – 0.05 = .95.

Step 5: Look up the area from Step in the z-table. The area is 1.645. This is your critical value for a confidence level of 90%.

## Common confidence levels and their critical values

You don’t have to perform the above calculations every time. This list of critical values and their associated two-tailed test confidence levels were calculated using the above steps:

## Find a Critical Value: Two-Tailed Test

Watch the video or read the steps below:

**Sample question**: Find the critical value for alpha of .05.

**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 answer from Step 2 in the middle section of the z-table*. The fastest way to do this is to 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 the plus or minus sign! In our example you’d get **±1.96**.

## Find a Critical Value: Right-Tailed Test

**Sample question**: Find a critical value in the z-table for an alpha level of 0.0079.

**Step 1:*** Draw a diagram, like the one above. Shade in the area in the right tail. *

This area represents alpha, α. A diagram helps you to visualize what area you are looking for (i.e. if you want an area to the right of the mean or the left of the mean).

**Step 2:** *Subtract alpha (α) from 0.5*.

0.5-0.0079 = 0.4921.

**Step 3:** *Find the result from step 2 in the center part of the z-table. *

The closest area to 0.4921 is 0.4922 at z=2.42.

That’s it!

Like the explanation? Check out the Practically Cheating Statistics Handbook, which has hundreds more step-by-step solutions, just like this one!

## Find a Critical Value: Left-Tailed Test

**Sample question**: find the critical value in the z-table for α=.012 (left-tailed test).

**Step 1:*** Draw a diagram, like the one above. Shade in the area in the left tail (because you’re looking for a critical value for a left-tailed test). *

This area represents alpha, α.

**Step 2:***Subtract alpha (α) from 0.5*.

0.5 – 0.012 = 0.488.

**Step 3:** *Find the result from step 2 in the center part of the z-table.** *

The closest area to 0.488 is at z=2.26. If you can’t find the exact area, just find the closest number and read the z value for that number.

**Step 4:***Add a negative sign to Step 3 (left-tail critical values are always negative)*.

-2.26.

*That’s it! *

Like the explanation? Check out our statistics how-to book, with a how-to for every elementary statistics problem type.

Uhmm….followed the instructions, looked at my Z chart and did not find step 4. My z chart gets me -0.07….what gives? where did I go wrong?

Are you looking at the right z-chart for your problem? For example, left of z or right of z.

YOU HAVE MADE ME UNDERSTAND STATISTICAL CRITICAL POINT

I cannot figure this out for the life of me! The table isnt working for me. This is my question: Compute the critical value z a/2 that corresponds to a 94% level of confidence.

Hi, Kristie,

Thanks for dropping by. Can you ask your question on the forum? One of our mods will be happy to help you with any statistics question :)

statisticshowto.com/forums

Very useful! Thanks a ton!

Hi,

I am confused. Checking the Ztable for teh critcal value when the level of significance is 0.05 like stated in step 1 and two. I am seraching in the Z table and find the value +/- 1.6 instead of 1.9. What do I forget or dont understan looking up for the critical value in the z table?

Thanks, Bas

Bas, it’s hard to say without seeing exactly what you’re doing. One thought — are you sure you are using the correct z-table? That’s usually why students come up with a different number…

How can i use scientific calculator to get this answer

How to find a critical value on a TI83

Regards,

Stephanie

Best explanation I have come across for determining where the critical value of 1.96 comes from – thanks.

I came up with 0.0 on the left column and .06 on the top of the z-table I’m I missing something here. Please help.

Victor,

Are you looking at the right z-table? If you find .475 using the search function, you should see it. The 0.0 row has VERY tiny values, so it makes me think you might be looking at a different z-table in your textbook? (Some texts, especially those for biological sciences, will have z-tables for very tiny incremenets).

Stephanie

Im not sure whats happening here, but this isnt the correct way to find those values. You just have to divide .05 by 2 to get .025. Subject .025 from 1 (1-.025) to get .975. THEN you search the Z Table for .975 to come to the 1.96. Either I am missing something, which may certainly be the case, or your work on this is just way wrong (respectfully).

Hello, Bernard,

Thanks for your comment. The technique here assumes you’re using a standard z-table (right of curve). This table only goes up to .5 (which makes sense because the area under the curve is 1). So it wouldn’t be possible to find .975 in a standard (right of curve) table. You have to divide the alpha level by two when using the standard z-table, because of the two tails.

If you’re using the FULL z-table, THEN your method would work to come to the same answer — 1.96.

I guess it all depends on which method your instructor chooses to use. As with many cases in math, there’s always more than one way to do things :)

I hope that helps!

Stephanie

Wouldn’t you divide .05 by 2 to get .025 then subtact that from which will give you .975 or as shown on the z chart 1.96

As with many things in math, there’s more than one way of getting the “correct” answer “)

Best,

Stephanie