The window trap: unequal histories make rankings lie

The ranking that flips

Across 2000–2026, Tesla has the highest full-available CAGR at 40.9%. It is tempting to call it the best compounder. But Tesla’s series begins in 2010, while Nvidia’s begins in 2000. Put the top names on the same window — from 2012-05-18, the latest common inception among them — and the leader flips: Nvidia 59.5% versus Tesla 46.0%.

Why it happens

CAGR annualizes whatever window it is handed. A late starter that compounded through one spectacular run will out-rank a long-horizon compounder that also had to live through the dot-com crash and the 2008 financial crisis. So a “performance ranking” built on full-available histories is, in part, a ranking of start dates — an accident of when each series happens to begin.

The discipline

Never compare full-available rows directly. Choose a common window — the latest inception among the assets you are comparing — and re-rank there. Report both columns, and label the full-available one explicitly as not equal length. The point is not that one number is wrong; it is that the two numbers answer different questions, and only the equal-window one is a fair comparison.

The general point

The same trap recurs everywhere a headline depends on a window: a risk-free series that starts late flatters Sharpe; a crisis window drawn a week early turns a crash into a gain; a currency lens reweights every local return. A headline number is only as honest as the window it is computed on — so state the window, every time.