Distance
========

* Has natural zero point (no distance no travel).
* You cannot travel a negative distance.
* 2m to 4m is twice the distance.

Hence, distance is a ratio data.

Displacement
============

* Has a zero point (stayed or ended up in the same place).
* Can be negative (back 2m).
* Can we talk about 'twice the displacement'?

So it seems that displacement is interval data. 

Is that right?