Different data sets will have different means and standard deviations, so values from one set cannot always be compared directly with those from another. The zscore standardizes normally distributed data sets, allowing for a proper comparison and a consistent definition of percentiles across data sets. Zscores can be found in a table, but using a TI84 Plus calculator is easier and more accurate. There are two methods of finding the zscore of a data set: you can use the TI84 Plus to find the mean and standard deviation and then use the zscore equation, or you can use the TI84 Plus invNorm function with a percentile as the argument.
Things You'll Need
 TI84 Plus calculator
 Data set
Use the ZScore Equation

Store your data set in the TI84 Plus as a list by pressing STAT and then selecting 1:Edit from the menu. The screen will change to show the current lists and an entry line at the bottom of the screen. If less than three lists exist in the calculator memory, the screen will show empty columns.

Move the cursor to an empty list using the arrow keys if the list currently selected is not empty. Add each data point into the list by typing the value on the entry line and pressing ENTER.

Calculate the mean and standard deviation of the data set. Press STAT and then the right arrow key, which will bring up the menu for statistics calculations. Press 1 and then press ENTER to select 1Var Stats. The screen will change to show the list name and the word CALCULATE.

Make sure the list name shown is the same list in which you entered your data. If it is not, use the arrow keys to move to the list name and type in the correct list. Leave FreqList blank.

Move the cursor to the word CALCULATE with the arrow keys and press ENTER. The screen will change again to show several statistics parameters, including the mean and standard deviation. Record these two parameters to use in the zscore equation.

Calculate the zscore by subtracting the mean from any data point in your list and then dividing that answer by the standard deviation.
Use the invNorm Function

Press 2ND and then VARS to display the DISTR menu. Select 3 and press ENTER to bring up the invNorm wizard screen.

Enter the desired percentile as a decimal next to the word area. For example, to find the zscore associated with the 95th percentile, type 0.95. Use the arrow keys to select Paste and then press ENTER.

Press Enter again, and the TI84 Plus will calculate the zscore associated with the chosen percentile.
Tips & Warnings
 These instructions also work for the TI84 Plus Silver Edition calculator.
References
 Laerd Statistics: Standard Score
 Ohio State University Department of Statistics: How To: Use the Normal Distribution Using the TI84
 Tompkins Cortland Community College: Normal Calculations on TI83/84 or TI89 for Individuals and Samples
 TI84 Plus and TI84 Plus Silver Edition Guidebook; Texas Instruments
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