Tracking error
How far do a portfolio's returns drift from the benchmark it claims to follow?
In plain words
Tracking error is the spread between a portfolio's returns and those of its benchmark, measured as a standard deviation of the difference over time. A small tracking error means the portfolio closely follows the benchmark; a large one means it strays.
Why it matters
Tracking error reveals how much extra risk a strategy takes relative to its reference index. When it widens, the portfolio is carrying bets that are diverging from the benchmark, so returns can swing far from what an investor expected from the index alone.
In a real week: BTC, week of 5 May 2025
In the week of 5 May 2025, BTC returned 10.44%, while ETH outpaced BTC by 28.57 percentage points, ending the week as the larger mover. Two assets moving in the same direction yet separated by that much is a concrete instance of divergence: a portfolio tracking BTC against an ETH benchmark would show a wide weekly tracking error.
A common mistake
Treating tracking error as purely unwanted. It can also signal deliberate active positioning. What matters is whether the deviation is intentional and understood.