A percentage rate-of-change of a time-series
at a particular point in time,
is calculated by
dividing the amount that the series has changed over the last
`n`-periods
by the value `n`-periods ago.
The result is the percentage that the value has changed in the last
`n`-periods.

Calculating:

```
R
```_{i} = (x_{i} - x_{i-n}) / x_{i-n}