My previous answer was just too convoluted (and it didn't actually compute what OP wanted). It appears there's a much simpler way to do it:
#This is actually the way to do it
This is actually the way to do it
| Date | sleep | productivity | determination coefficient |
|-------+-------+--------------+---------------------------|
| [meh] | 7.50 | 4.00 | |
| [meh] | 8.00 | 5.00 | |
| [meh] | 8.50 | 6.00 | |
| [meh] | 8.00 | 4.00 | |
| [meh] | 8.20 | 5.00 | |
| [meh] | 8.70 | 6.00 | |
|-------+-------+--------------+---------------------------|
| | | | 0.89847792 |
| | | | 0.89847792 |
#+TBLFM: @9$4=vcorr(@2$2..@7$2, @2$3..@7$3)
#+TBLFM: @8$4=vpcov(@2$2..@7$2, @2$3..@7$3)/(vpsdev(@2$2..@7$2)*vpsdev(@2$3..@7$3))
As suggested by the comments. Note that this gives you r, whereas RUserPassingBy's results give r^2 (but, finally, both results are correct).