Yup, this one again:
|The big one|
I don't really have a reason for picking a "linear" trendline over one of the curved ones. I picked linear because that's the one I always see. Don't remember many curved trendlines on scatterplots. The only one I can remember is the Phillips curve, and things didn't work out so well for that one.
I've long felt there must be an optimum range for the P2P ratio, one that best promotes economic growth. And a curved trendline might show such a range, where the linear cannot. So I decided to look at some trendlines. Note: I can't tell you anything technical about trendlines. (Maybe you can tell me?)
I wonder what curve calculation old Bill Phillips used...