DeGiro vs the S&P 500 or MSCI World: comparing fairly
The index's own return and a fair comparison to it are not the same number, and the gap between them is entirely about timing.
Why the obvious comparison is unfair
The index's quoted return for the year assumes one thing: money in on day one, left alone until the last day. That's almost never how a real account works — deposits arrive through the year, in whatever amounts and on whatever dates suited you at the time. Comparing your account's return against the index's headline number is really comparing your actual, staggered deposits against a hypothetical lump sum that was never actually available to you on 1 January. If the index happened to fall early in the year and recover later, a lump-sum investor eats the whole drop and a gradual one doesn't — and that's true even if both of you are otherwise doing exactly the same thing.
What a fair comparison actually does
Take your own deposits and withdrawals — the real ones, on the real dates — and simulate putting that same money into the index instead, on those same dates, marked to market every day after. That produces a benchmark value series built from your own cash-flow history rather than from an assumption, so the two lines on the chart started from the same money at the same times and can be read against each other honestly.
The comparison this tracker draws on the Overview tab is built exactly this way — your deposits, your dates, your chosen index — rather than as the index's own quoted return sitting next to your account's.
A worked example
Say an index stands at 100 on 1 January, falls to 80 by 1 April and ends the year at 104. Its headline return for the year is 4%. You put €3,000 into your account on 1 January and another €3,000 on 1 April, and by 31 December the account is worth €6,900 — 15% more than the €6,000 you put in.
Against the headline, you beat the index by 11 points. Now put the same money into the index on the same dates: €3,000 at 100 buys 30 units, €3,000 at 80 buys 37.5 more, and 67.5 units at 104 are worth €7,020. The fair benchmark ended €120 ahead of you. On a money-weighted basis the index money made 19.6% a year against your 17.3%.
So the same year reads as “beat the market by 11 points” or “trailed it by 2.3 points”, depending only on whether the comparison respects when your money went in. The second answer is the true one: your second deposit caught the index's dip, and the index would have caught it too.
Price index or total return
The index level quoted in the news is usually a price index: it tracks share prices and leaves out the dividends the companies paid. Your account is not a price index — every dividend you received is in your cash balance, part of what the account is worth. Comparing your account with a price index hands you the dividends and denies them to the index, a head start of roughly the index's dividend yield every year, typically a percentage point or two for broad equity indices.
A fair comparison uses the total-return version, with dividends reinvested. That is what the tracker's benchmark does: the simulated index holding has its dividends reinvested, just as your own account is credited with yours.
The currency the index is in
The S&P 500 and the Nasdaq-100 are priced in dollars; an account in euros is measured in euros. If the dollar weakens against the euro over the year, a euro investor in the S&P 500 earns less than the index's dollar return, and the other way round. A fair benchmark converts the index into your currency at each day's exchange rate, so it carries the same currency effect your own US holdings carry. The currency guide works through how big that effect can be.
Picking an index to compare against
The S&P 500 is the 500 largest US companies; an all-world tracker adds developed and emerging markets outside the US; the Nasdaq-100 is tilted hard toward large US technology companies and is a noticeably more concentrated bet than either of the other two. None of the three is automatically “correct” to compare against — it depends what you'd have put the money into if not into the positions you actually hold, and a portfolio concentrated in one sector or region is fairly compared against something that shares that concentration, not against whichever index has done best lately.