[R] OT: computing percentage changes with negative and zero values?

[Jim Lemon]

On 02/17/2010 04:06 AM, Liviu Andronic wrote:
> Dear all
> I need to compute percentage changes of my data, but unfortunately
> they contain both negative and zero values, and I am quite confused on
> how to proceed. Searching the internet I found that many people ran
> into similar issues, with no obvious solution available.
> ...
> As you have noticed, I'm quite unsure on how to proceed. My actual
> data represents financial EPS (earnings per share) forecasts, ranging
> from -1 to 5. So, it has a "natural zero point"  (see David Winsemius'
> comments in [2]). However, I need to compute percentage variations
> since I am primarily interested in the evolution of the forecasts (for
> a given company), while EPS data between two companies are not
> necessarily comparable. The percentage data would subsequently be used
> in performing statistical analyses (regression, etc.).
>
Hi Liviu,
My understanding of percentage change is the absolute value of the 
change divided by the absolute value of the initial value and the result 
multiplied by 100. Thus:

100*abs(diff(x$var))/abs(x$var[1:19])
  [1] 283.720930 187.341772  10.144928 100.000000        Inf  52.980132
  [7] 212.676056  46.250000  35.042735   6.329114 223.648649  51.912568
[13] 263.636364 150.000000  69.444444 959.090909 167.195767  40.157480
[19] 275.000000

gives me correct values (and the Inf for the fifth value is correct, as 
the denominator is zero). My definition might not be the only one, though.

Jim