In a nutshell, this is how I approach investing.
It's a tool professionals use to speed up how fast their returns compound, and it's actually very simple.
Start with the Nasdaq-100: the hundred largest companies on the Nasdaq, weighted by size. In practice, that makes it a list of the biggest technology businesses on earth.
Technology gets adopted. That's the whole reason for the line that runs up and to the right. The Nasdaq has compounded at roughly 14% a year since 1985, and lately even faster: over 20% a year across the past decade, and almost 25% a year over the past three. It isn't a perfect stand-in for any single company, but as a read on how the market is pricing technology as a whole, it's as good as exists.
Here's how the tool works. It fits a straight line through that forty-year history, on a scale where every doubling takes up the same space. That line is the trend. The index tends to run ahead of it when investors are excited, and fall behind it when they're frightened. A standard deviation is just the average size of that gap.
From there, three simple zones.
Fair means the index is sitting inside the band. Invest on schedule.
Expensive means it's more than one band above the trend. Keep contributing monthly if that's your habit, but this is not the moment to deploy large amounts of capital.
Cheap means it's more than one band below. Historically, this has been the entry point, and historically, it's the uncomfortable one, because the reasons it's down there are always splashed across the news.
That last zone is the entire point of the tool. The same dollar buys more of the index below the trend than above it. This is where I add my own capital, and the more oversold it gets, the more aggressive I tend to be. How you approach it is your business. The tool simply exists to give you an edge in how you compound.
One important caveat. The tool says nothing about when the gap will close, and nothing about any individual company. It tells you, broadly, where US technology is standing today. That's all. It's updated monthly.