Statistics How To

How to Find Q1, Q3 and the Interquartile Range TI 89 calculator

TI-89 statistics > Interquartile Range TI 89

How to Find Q1, Q3 and the Interquartile Range TI 89 calculator

The interquartile range(IQR) (sometimes called the midspread or middle 50), is a statistical measure of dispersion. It’s the distance between the 25th percentile and the 75th percentile. It’s commonly used to get an idea of where the bulk of scores lie. The average, or mean, tells you where the center point is for a set of scores, but the IQR tells you what the wider range is around that center. For more on the IQR and what it’s used for, see What is an Interquartile Range?

In order to calculate the IQR by hand, you would have to do a few separate calculations, including finding the mean. The interquartile range formula is Q3-Q1, where Q3 is the third quartile (75%) and Q1 is the first quartile (25%). You could use the online interquartile calculator found on this site. However, you can’t bring the internet to a test, and it’s easy to make errors when calculating by hand. That’s where the TI-89 calculator comes in handy.

Sample problem: Find Q1, Q3, and the IQR for the following list of numbers: 1, 9, 2, 3, 7, 8, 9, 2.

Step 1: Press Apps. Scroll to Stats/List Editor (use the arrow keys on the keypad to scroll). Press Enter.

Step 2: Clear the list editor of data: press F1 8.

Step 3: Press Alpha,9 Alpha,1, and Enter. This names your list “IQ.”

Step 4: Enter your numbers, one at a time. Follow each entry by pressing the Enter key. For our group of numbers, enter
1 , 9 , 2 , 3 , 7 , 8 , 9 , 2

Step 5: Press F4, then Enter (for the 1-var stats screen).

Step 6: Tell the calculator you want stats for the list called “IQ” by entering Alpha,9 Alpha,1 into the “List:” box. The calculator should automatically put the cursor there for you. Press Enter.

Step 7:Read the results. Q1 is listed as Q1X (in our example, Q1X=2). Q3 is listed as Q3X (Q3X=8.5). To find the IQR, subtract Q1 from Q3 on the Home screen. The IQR is 8.5-2=6.5.

That’s it!

Tip: You could copy and paste on the TI 89 (see manual) but the steps are going to take you more time than just writing down the numbers and subtracting on the home screen. If you do want to copy and paste:
1. Highlight the expression on the Home screen. Press ƒ1 and select 5:Copy.
2. Display the Y= Editor, highlight the desired function, and
press Enter.
3. Press ƒ1 and select 6:Paste. Then press Enter.
You could also use the diamond keys to copy and paste.

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