Tracking difference vs tracking error: the metric that matters is not the TER

You pick an index fund by its TER and assume that is what it costs you. Not quite. The figure that really measures what you lose against the index is a different one, it is rarely published, and it almost never matches the TER.

The two metrics, and why they get confused

• Tracking difference: how far the fund landed below — or above — its index over a period. If the index did 10% and the fund 9.85%, the difference is 0.15 points. This is the one that costs you money.
• Tracking error: the standard deviation of that difference. It measures whether the fund drifts steadily or in jumps. It measures consistency, not outcome.

A fund can have a tiny tracking error and still lose half a point a year, very consistently. Good aim, wrong target.

Why it does not match the TER

Because the TER is only one of the terms. What really separates a fund from its index:

• Dividend withholding tax. The largest term, and almost always invisible. It depends on the fund's domicile and where the companies sit.
• Securities lending income. This works in your favour: it can give back 0.02 to 0.10 points a year, sometimes more in small-cap indices.
• Sampling. If the fund does not buy the whole index, the selection drifts a little either way.
• Transaction and rebalancing costs. Every index review forces buying and selling, which carries cost and slippage.
• Uninvested cash. Money waiting to be deployed does not earn the index return.
• The replication method. A synthetic ETF can avoid part of the withholding tax, which improves its tracking difference.

Yes, a fund can beat its index

And it is neither a calculation error nor closet active management. If securities lending income and tax savings outweigh the TER, tracking difference comes out positive. It happens fairly regularly on synthetically replicated US indices and in funds that lend aggressively.

Careful though: that is not free money. It means you accepted some counterparty risk in exchange.

How to check your own fund's

Nobody advertises it, but you can work it out:

1. Find the fund's return and the index's over the same period, in the annual report or the provider's monthly factsheet. They usually sit side by side.
2. Make sure you compare the same index version. It must be Net Return, not Price Return — otherwise the difference you compute is wrong by about 2 points.
3. Subtract them. That is your tracking difference for the period.
4. Look across several years. A single year can be distorted by the dividend calendar or a large index review.

What to do with this

Do not turn it into an obsession. The sensible order is still: choose the index first, rule candidates out by fund size and structure (accumulating or distributing), and only then fine-tune on cost. Tracking difference is the tiebreaker between two candidates that already suited you, not the first criterion.

And if you are comparing funds tracking different indices, this metric is useless: they are not comparable. That is a different question, covered in how to choose an index fund.

In short

• Tracking difference is what you lose against the index; tracking error only measures how steady that loss is.
• The TER is one term, not the total: dividend withholding and securities lending weigh more.
• A fund can beat its index, usually through tax savings or securities lending.
• Always compare against the Net Return version of the index, or your figure is wrong.

Educational content, not investment advice.