This tutorial demonstrates how to generate and interpret the numbers produced by a TI-84 plus graphing calculator when attempting to find a regression line or “line of best fit” from a set of data. This is especially important for statistics students, who will be required to use this calculator function as part of their class. It is also important in any situation where a linear function must be fitted to a set of data.
Part 1 Creating the Formula For the Regression Line
1. Power on the calculator. Press the ON button at the lower left-hand corner of your calculator.
2. On the calculator, press the STAT button. The word EDIT should appear at the top of your calculator’s screen, and the word “1: Edit” should be highlighted. At the bottom right-hand corner of the calculator, press ENTER.
3. Enter your values. There should be two columns, one labelled L1 and the other L2. This is where you will enter all of the values from your data set that you should have ready to go. Under L1, enter your X values or first set of values. Fill in the blanks with your Y or second set of values. Enter a value, then press ENTER and repeat.
4. To exit the table of values you’ve created, press the 2ND and then MODE buttons. These values will now be saved in your calculator as variables L1 and L2, which stand for list 1 and list 2, respectively.
5. Press the STAT button once more. Press the right arrow until the word “CALC” appears at the top of your calculator’s screen. Press the down arrow three times until the phrase “4: LinReg(ax+b)” is highlighted, then press the ENTER key.
6. Make sure the X-list option is selected. Under “4:LinReg(ax+b),” Xlist should already be selected and waiting for a list of data. This is where you will enter your X values, also known as L1 data values. Press the 2ND button, followed by the 1 button. This should result in L1 appearing next to the colons. When you press the down arrow once, the Y-list should be highlighted. Press the 2ND button, followed by the 2 button. The L2 value should now appear next to those colons.
7. Compute these values. At the bottom of the calculator’s screen, press the down arrow until “Calculate” is highlighted. Once this has been highlighted, press the ENTER key.
Part 2 Interpreting the Values
1. Make certain that you have completed Part 1 correctly. If you did, four variables should appear on your screen. These are a, b, r, and r2 (which stands for exponent). Each of these variables has a distinct meaning in terms of the interpretation of your line.
2. Determine which values are most important. The a and b values are the most important for the actual line. These can be entered into the equation y=ax+b, which appears at the top of your calculator, to obtain the regression line or line of best fit for your data set. The slope of your function is represented by the a variable. The y-intercept of the graph is represented by the b variable.
3. Recognize the other two values. The two other values you’re interpreting, r and r2, are there to assist you in interpreting the fit of your given line to the data set you provided. The r value indicates the degree of correlation between your X and Y variables. Your r2 value is the correlation coefficient r squared, and it indicates how well your line fits the data you provided.
Part 3 Troubleshooting (optional)
The following steps only apply if your r and r^2 values don’t show up.
1. Press the 2ND button and then press 0. This will take you to a menu with the title “CATALOG”. It should have a long list of commands for your calculator.
2. Press the down arrow until the arrow on your screen indicating the command you want your calculator to perform is set to “DiagnosticOn.” Press the ENTER key twice.
If you now go back and repeat steps 5-7 from Part 1, your r and r2 values should appear.
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