If you have actually a inquiry asking you to discover z alpha/2, you’re being asked to uncover an alpha level’s z-score for a two tailed test.

Alpha level are pertained to confidence levels: to discover alpha, just subtract the confidence interval from 100%. For example, the alpha level because that a 90% to trust level is 100% – 90% = 10%.

To find alpha/2, divide the alpha level through 2. For example, if you have actually a 10% alpha level then alpha/2 is 5%.

You are watching: Compute the critical value za 2

How to uncover z α/2

You have three main options:

Use recognized values because that z alpha/2.Use a z-table.Use the TI-83/84.

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1. (Easiest Way) Use well-known values for z alpha/2.

You don’t actually need to look increase z alpha/2 in a z-table every time. For many hypothesis tests, you’ll most likely be making use of one of 4 confidence levels (90%, 95%, 98% and also 99%). The z alpha/2 because that each confidence level is always the same:


Note: This step relies on using the right-hand z-table ~ above this site. Over there are several different possible z-table layouts, so girlfriend may acquire a different answer if you use a various z-table. I would suggest using the very same z-table when learning just how to look increase areas/z-scores until you acquire the cave of it…then girlfriend should be able to use any type of z-table friend come across.

TI-83 / TI-84

Step 1: press “2nd” and also then push “VARS”.

Step 2: select “invNorm” and also then press “ENTER.”

Step 3: type in the portion for alpha/2 native the above table. For example, kind in 0.005 for a 99% to trust level.

Step 4: kind a close up door parentheses “)” and also then push ENTER.

The an outcome will be the z-score for the left tail (in this example, -2.576). As the normal distribution curve is symmetrical, the cutoff because that the ideal tail is the opposite: 2.576.


Gonick, L. (1993). The Cartoon guide to Statistics. HarperPerennial.Everitt, B. S.; Skrondal, A. (2010), The Cambridge thesaurus of Statistics, Cambridge college Press.

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