How to calculate average growth when it's negative? We have annual reports for company's revenue and can calculate annual growth as 
$yg = {y_{i+1} \over y_i}$.
And then we can calculate the average monthly growth as $mg = ({y_{i+1} \over y_i})^{1 \over 12}$.
So for reports 2000-12 $1m and 2001-12 $2m the average monthly growth would be 1.06.
But how calculate monthly growth when the revenue became negative?
For reports 2000-12 revenue = $1m and 2001-12 revenue = $-1m?
P.S.
I need it for simple prediction. For example 2000-12 $1m and 2001-12 $2m  the revenue in 2002-02 could be predicted as $2 \times 1.06^2 = 2.25$

An example for negative growth rate: $y_0=100, y_1=80$ 
The growth rate from $t=0$ to $t=1$ is $g_{01}=\frac{80}{100}-1=0.8-1=-0.2$
So you can use the formula for growth rate no matter whether the growth rate is positive or negative:
$$g_{t,t+1}=\frac{y_{t+1}}{y_t}-1$$
Btw, the growth factor $1+g_{01}$ is still positive $1-0.2=0.8$ 

To apply the formula for the growth rate you need a meaningful zero point. That means that the values are ratio scaled. If $y_t$ is can  be negative as well, then a growth rate cannot be determined.
