Question: how to calculate the correlation coefficient on ti 84 plus?
Answer: To view the Correlation Coefficient, turn on "DiaGnosticOn" [2nd] "Catalog" (above the '0'). Scroll to DiaGnosticOn. [Enter] [Enter] again. ... 
Now you will be able to see the 'r' and 'r^2' values. Note: Go to [STAT] "CALC" "8:" [ENTER] to view. Prev: TI-84: Least Squares Regression Line (LSRL)