Catching a growth trend variable for 5 yearly sales number

[raulz]

Hey everyone,

I'm doing research in international business. I will skip the details and get to my basic problem directly.

I have 300 firms in my sample, for which I have 5 yearly numbers of total sales.

For each firm I would like to calculate a variable that catches the trend in (negative) sales growth. I need a (trend) variable for sales growth in a regression analysis.

Visualizing this one can easily plot trend lines in a spread diagram in Excel. These graphs express the trend I want to translate into a variable that allows me for comparison between firms.

Is there any formula?

I will give an example for one firm:

Sales between 2005-2008:
2005: 193145
2006: 154512
2007: 448355
2008: 524282
2009: 586035

trend line formula: y=115555x +34601 (where y=total sales and x=time (t1,t2.. etc)

I know that it is not difficult to calculate a steepness variable of the lineair graph (íf you know two couples of x and y, but I can't catch these). I can't find out how in Excel. Neither in SPSS.

Besides it would be way easier if I could use a formula, so I won't have to calculate each variable independently.

THANK YOU IN ADVANCE!