A critical value in hypothesis testing separates the region where the hypothesis will be rejected from the region where the hypothesis will not be rejected. With a little arithmetic, you can look up a critical value in a z-table, or you can use the InvNorm function on the TI-83 graphing calculator.
Sample Problem: An end of semester exam is normally distributed with a mean of 85 and a standard deviation of 10. Find the z score at the 90th percentile.
Step 1: Press 2nd VARS 3. This displays InvNorm( on the home screen.
Step 2: Type . 9 0 , 8 5 , 1 0.
Step 3: Press the ) button.
Your display should read InvNorm(.9, 85, 1, 0).
Step 4: Press ENTER. This returns 97.82. That means that 90% of students will have scores below 97.82.
That’s it! Like the explanation?
Tip: The first entry on InvNorm should be a number between 0 and 1.