Modeling Viral Growth and Marketing in PowerPivot

Let's say you operate a business that relies heavily on "word of mouth" - customers recommending your product/service to their friends and colleagues. Or at least, you THINK it relies heavily on that sort of thing.

You need to decide how much to spend on traditional advertising - to supplement the social/viral marketing that your customers do on your behalf. Take a look at each of these two charts - the captions for each attempt to capture the knee-jerk conclusions you might draw: