Tutorial for calculating ‘a’ and ‘b’ values in Casio FX991-ES
Step 1: Press “mode” and the go to ‘STAT’ mode (press 3)
Step 2: Select 2 (i.e. A+BX), since we are carrying out linear regression analysis
Step 3: Now we have enter the values. I will explain you the method with these sample values
X Y
1 6
2 7
3 8
4 9

Under the column, ‘X’, press 1 and then press ‘=’ and similarly for entering 2 (press “2” and “=”) and so on. Now for entering values of ‘Y’ use the navigation button (the blue circular button at the top) to get to column ‘Y’. Similar to the process followed for entering values of ‘X’ follow it for ‘Y’( press ‘9’ and ‘=’…..)
Step 4: Now to calculate values of ∑▒X , ∑▒Y, ∑▒X Y press ‘shift’ and then now press ‘1’ to get the menu.
Note: If you want to check your data, you can do it by selecting ‘Data’ (that is ‘2’)
Step 5: Press ‘Sum’ (i.e. 4). Now you will have another menu where you can calculate all the values of ∑▒X, ∑▒Y, ∑▒X Y etc. Press the corresponding number to calculate its respective value.
For example: If you want to calculate the value of ∑▒X Y, press ‘5’ .
Step 6: Press ‘AC’. Now, to calculate the values of ‘a’ and ‘b’ press ‘SHIFT’ and the press ‘1’ to get the menu .
Step 7: Select ‘Reg’ (i.e. 7). To calculate the value of ‘a’ press ‘1’ and similarly for ‘b’, press ‘2’.
Step 8: Now you have the final regression equation, Y=5+b, with values of (a, b) = (5, 1).

Tutorial for calculating ‘a’ and ‘b’ values in Casio FX991-MS
Step 1: Press “mode” two times and the go to ‘REG’ mode (press 2).
Step 2: Select ‘Lin’ (i.e. 1), i.e. linear since we are carrying out linear regression analysis.
Step 3: Now we have enter the values. I will explain you the method with these sample values
X Y
1 6
2 7
3 8
4 9

Here, we enter the value of ‘1’ and its corresponding value, ‘6’ at the same time. For this, press 1, 6 (you have to enter the comma) and then press ‘M+’ button. For the next set of values, press 2, 7 and then ‘M+’. Similarly, for all others too.
Step 4: Now to calculate values of ∑▒X , ∑▒Y, ∑▒X Y press ‘shift’ and then now press ‘1’ to get the menu. To calculate the value of ∑▒X, press ‘2’. If you want to calculate the value of ∑▒X Y, use the navigation key (the grey button at the top) to go to the right and then press ‘3’.
Step 5: To calculate the values of ‘a’ and ‘b’, press ‘SHIFT’ and then ‘2’. Now use the navigation key to go towards the right(press twice) and then press ‘1’ to calculate the value of ‘a’ and then ‘2’ to calculate the value of ‘b’.
Step 6: Now you have the final regression equation, Y=5+b, with values of (a, b) = (5, 1).
Note: For older casio scientific calculators the method mentioned above, would most probably work.