Something I have been looking at since the early 90's has been the "velocity of money" and the "multiplier effect". The most basic definition of the velocity of money is the number of times $1.00 is spent to buy goods and services per unit of time. Economic theory claims the higher the number, the stronger and more productive the economy. Importantly, a higher velocity of money usually means that inflation should be higher too.
I will refrain myself from going into a lot of detail here but for the past 8yrs, folks (monetarists) have been saying that because central banks have increased the money supply, inflation would resurrect it's ugly head. Well, clearly, that has not happened. Why not? Because of two main reasons:
Let's look at a chart on this. Below is the US 30yr Treasury Bond in yellow dating back 25yrs. The blue line is just a simple trend line that I drew in (this is just based off of closing levels but over 25yrs that is probably ok). The white line is the velocity of money. And the green line is the St Louis' Fed's Multiplier. Pretty high correlation between the three isn't it?
Almost makes me want to go and buy back some of the 30yr bonds we sold in Sept.