I'm doing some work in a domain where it is normal to say that if the value was -1 and is now 2 There has been a +300% change.
To give some other examples:
( 10, 5) -> - 50%
( 10, -5) -> - 150%
(-10, 5) -> + 150%
(-10, -5) -> + 50%
Ie. if the value was 10 and it's now 5, there has been a 50% decrease. If the value was 10 and it's now -5, the change has been a 150% decrease. Etc.
The equation is: $$\frac{\text{value_latest} - \text{value_previous}}{abs(\text{previous_value})}$$
Does this calculation have a name? I am trying to research some of it's properties but I'm not sure what to google.
Perhaps ‘change relative to the absolute value’, or ‘relative error’ (as opposed to ‘relative change’) ?
Wikipedia: http://en.wikipedia.org/wiki/Relative_change_and_difference (sections 2 & 4).