Getting 25% growth over last year sale

Question

I want to calculate 25% growth over prior year's sale (by month). The way I am solving this currently is by multiplying (sale)*1.25; this works fine for sale values that are positive but for those that are negative I get that the 25% growth is more negative than the the sale amount. I don't want to tell the salesperson that I want them to sale even less than they have sold in the prior year.

Example:

Prior Year Jan - 40000 Feb - -20000 (negative because they can have sales taken away)

Current Year - I want the salesperson to have a growth of 25% over their prior year amount Jan - 40000*1.25 = 50000 Feb - -20000*1.25=-25000 (based on my calculation they should actually be at -15000, this is 25% growth from their -20000)

I am trying to find a solution that works for both scenario as it needs to go into a equation/formula.

Thank you very much for your help!

Answer 1

Mathematically speaking, a 25% growth will always be found by multiplying by 1.25. It looks like you're looking for a 25% improvement, so to speak, over the previous values. For what you want to do, a formula like $f(n) = n + 0.25|n|$ will suffice.

Answer 2

The problem is, that you have negative sales (whyever). Your data don´t have a zero point. Thus they are not ratio scaled. But this is the assumption to calculate a growth rate for every month.

I don´t really understand, why you have negative sales. This doesn´t sounds logical to me.