Key takeaways
- Physical ETFs hold securities; synthetic ETFs use a swap with a counterparty.
- Full replication is simplest; sampling trades some fit for lower trading costs.
- Synthetic risk centres on the counterparty and the quality of collateral.
- Tracking difference over several years is the best scoreboard.
- Read the factsheet: method, collateral, lending policy, swap providers.
Two ETFs can track the same index and hold completely different things inside. One owns the shares in the index. Another owns a basket of unrelated securities plus a contract with a bank promising to deliver the index return. That choice, called replication, shapes risk, cost and sometimes taxes.
Physical replication
A physical ETF buys real securities. There are two common styles:
- Full replication: the fund holds every constituent in the index at about the index weight. It is simple and transparent, and works best for liquid indices with a manageable number of holdings.
- Sampling (optimised) replication: the fund holds a representative subset chosen to behave like the whole index. It saves on trading costs when the index has thousands of small or hard-to-trade securities, at the price of a slightly looser fit.
Physical funds often lend out part of their holdings to borrowers such as short sellers and earn a fee. Part of that revenue is returned to the fund and reduces its net cost, but it introduces a risk that a borrower fails to return the securities. Lending is usually backed by collateral, and policies on how much can be lent differ between funds.
Synthetic replication
A synthetic ETF does not need to own the index constituents. It typically holds a basket of securities as collateral and enters a total return swap, a contract in which a counterparty, usually a bank, agrees to pay the fund the index return in exchange for the performance of the fund's basket.
This sounds exotic, yet it can be the practical option when the index covers markets that are expensive or difficult to reach directly, or when tax rules treat dividends from swaps more favourably. It can also lead to very close tracking.
The extra risk: counterparty exposure
The fund relies on the swap partner to deliver. If the partner defaulted, the fund would rely on the collateral. Regulated European funds known as UCITS are limited in how much unsecured exposure they may hold to a single counterparty, and the limit most often cited is 10% of the fund's assets. Many providers reset swaps regularly to keep exposure well below that.
| Feature | Physical (full) | Physical (sampled) | Synthetic (swap) |
|---|---|---|---|
| What the fund owns | All index securities | A representative subset | A collateral basket plus a swap |
| Tracking quality | Typically very close | Close, with some drift | Typically very close |
| Main extra risk | Securities lending | Sampling error, lending | Counterparty and collateral |
| Transparency | High | Medium-high | Requires reading collateral reports |
| Typical use | Large liquid indices | Broad, hard-to-trade indices | Hard-to-access markets |
Tracking difference: the scoreboard
Whatever the method, what counts is how closely the fund follows its index after costs. Tracking difference is the gap between fund and index returns over a period. Tracking error is how much that gap wobbles from day to day.
Take an illustrative year in which an index returns 8.00% and a fund with a 0.25% annual charge returns 7.80%. The tracking difference is minus 0.20%. It is smaller than the charge because securities lending income offset part of it. A different fund with the same charge and no lending income might show minus 0.25% or worse. Both are reasonable; the difference tells you who is running things efficiently.
Illustrative figures only, not data for any real fund.
What to check before you buy
A five-line factsheet review
- Find the replication method. The factsheet or key information document should state physical full, physical sampled or synthetic.Tip: If it is unclear, that is information too.
- For swaps, read the counterparties. Look up who the swap providers are, how many there are and how often the swap is reset.
- Inspect the collateral. Is the collateral basket liquid and diversified, and does it differ greatly from the index being delivered? Higher quality collateral means less worry.
- For physical, read the lending policy. Check how much can be lent, what collateral is held and what share of revenue the fund keeps.
- Compare the tracking history. Use several years of tracking difference and compare with similar funds.
Why the issuer may choose one or the other
Providers make a choice based on the index and on where the fund will be sold. A fund tracking a broad developed-market index often goes physical because the shares are liquid and cheap to hold. A fund tracking a market with restricted foreign access, or one that is tilted toward an unusual strategy, may use a swap. Knowing this helps you read the choice as an engineering decision rather than a warning sign, while still checking the details.
Which one is better?
Neither structure is automatically superior. Physical replication is easier to understand and has no swap counterparty. Synthetic replication can be more efficient where physical access is awkward. The question is whether you understand and accept the specific extra risk in return for the specific benefit.
Frequently asked questions
Is a synthetic ETF the same as a leveraged ETF?
No. Synthetic describes how an unleveraged fund gets index exposure. Leveraged funds use borrowing or derivatives to multiply daily returns and have their own risks.
Does securities lending make a fund unsafe?
It adds a small risk but is usually collateralised and capped. The key is whether the policy is transparent and a fair share of revenue returns to investors.
Why does my ETF's performance differ from the index?
Fees, trading costs, cash drag, sampling, lending income, currency timing and dividend tax treatment all contribute.