1) If we use the formula for calculating r, then we get $r = \frac{0}{0}$.

2) If we see r as a measurement of the strength and nature of the linear relationship between two variables, then we get $r = 0$ because there is no linear relationship in this case.

3) If a perfect positive fit gives $r = 1$ and a perfect negative fit gives $r = -1$, then it seems like we should get $r = 1$ or $-1$. (I think this is wrong though.)

Which is correct?

![enter image description here][1]


  [1]: https://i.sstatic.net/k81lY.png