How to Find a Critical Value for a Left-Tailed Test
Finding a critical value for a left-tailed test will have a familiar feel to it if you are used to looking up areas in a z-table (which should have been covered by this point in any elementary statistics class). This article shows you how to find α=.012 for a left-tailed test. If you like our easy to follow explanations of statistics, check out our easy to follow book, which has hundreds more examples, just like this one.
Step 1: Draw a diagram, like the one above. Shade in the area in the left tail.
This area represents α.
Step 2:Subtract alpha (α) from 0.5.
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.
That’s it! Like the explanation? Check out our statistics how-to book, with a how-to for every elementary statistics problem type.