Finding the First yDifferences of a Set of Data
x  1  3  5  7  9  11  13 
y  8  6.5  5  3.5  2  0.5  1 
When the xdifferences are all constant, then the ydifferences (or first differences) are useful in telling if a set of data is nearly linear or not.
To find the first ydifferences, press STAT, select 1:EDIT by pressing ENTER. Enter the x values in L1, and the y values in L2 as shown below.
Next, with the flashing cursor in the L3 column, press the UP ARROW once so that L3 is highlighted. Press 2nd LIST[STAT] and RIGHT ARROW to OPS menu.



From the OPS menu select 7: by pressing a 7. now appears at the bottom of the screen beside L3=. Press 2nd L2[2] to type L2, and then press the ) key. Press ENTER and the ydifferences will appear in the L3 column.



Examining the ydifferences in L3 column, we see that the data are all exactly equal to each either. This means that the data is exactly linear.