ETFs, Index Funds and Mutual Funds
A fund is a basket sold as one security. The differences that matter are how it trades, what decides its holdings, and what it charges.
MadStockAlerts Research · Updated August 28, 2026
What to take away
- An ETF trades intraday on an exchange; a mutual fund transacts once a day at its net asset value.
- Index and active describe how holdings are chosen, which is a separate question from structure.
- Creation and redemption is the mechanism that keeps an ETF near the value of its basket.
- The expense ratio compounds, and over long horizons it is one of the largest controllable costs.
- An index fund's real product is the rulebook of the index it tracks.
MAD Academy Training Video · 0:46
Three Wrappers, One Basket
ETFs, index funds and mutual funds can hold identical assets and still behave completely differently for the person holding them.
This lesson is part of a Stock Alerts + Tools plan.
Two separate questions
Fund terminology confuses two independent choices. The first is structure: how the fund is bought and sold. The second is mandate: how it decides what to hold. Any combination of the two exists, and conflating them produces most of the confusion in the category.
| Index mandate | Active mandate | |
|---|---|---|
| ETF structure | Most large ETFs; trades all day | A growing minority, including active bond and thematic funds |
| Mutual fund structure | Traditional index funds; priced once daily | The classic actively managed fund |
The phrase passive fund describes the mandate and says nothing about the wrapper. A passive index fund and a passive ETF tracking the same index hold the same securities and differ only in how an investor transacts in them.
How each one trades
An ETF has a bid and an ask like any listed security, so it can be bought at 10:14 a.m. and sold at 10:41 a.m., with all the usual consequences: a bid-ask spread to cross, and a price that can sit slightly above or below the value of the underlying basket.
A mutual fund does not trade. Orders placed during the day are all executed at the net asset value struck after the close, so every buyer and seller that day receives the same price and there is no spread to pay. The trade-off is that an order placed at 10:00 a.m. executes at a price nobody knows until the evening.
Neither is better in the abstract. Intraday tradability is valuable to somebody who needs it and is a cost to somebody who does not, because the ability to transact all day is also the ability to transact badly all day.
Creation and redemption
An ETF stays close to the value of what it holds because large participants can exchange a basket of the underlying securities for new ETF shares, or the reverse. If the fund trades above the basket, creating shares and selling them is profitable, and the act of doing it closes the gap.
This is a real arbitrage performed by real firms for real profit, and it is why an equity ETF's price tracks its holdings so tightly during ordinary conditions. Nobody is enforcing the relationship; it is enforced by the fact that a deviation is free money.
The arbitrage works well when the underlying is liquid and continuously priced. In corners where it is not, such as some high-yield credit or thinly traded international markets, an ETF can trade at a visible premium or discount for a while, especially under stress. The fund is not broken; the basket underneath it has become hard to price.
- 1Fund trades above the basketA premium appears
- 2An AP buys the underlying shares
- 3Delivers them to the fundReceives a block of new fund shares
- 4Sells those shares on the exchangeNew supply, at the premium
- 5Premium closes
- and back to the start
Costs
The expense ratio is deducted from fund assets daily and never appears as a line on a statement, which is what makes it easy to ignore. Over long holding periods it is one of the few costs entirely within an investor's control.
value retained over N years = (1 - expense ratio) ^ N
- at 0.75% a year, roughly 7% of the ending value is gone after a decade
For an ETF the true cost is the expense ratio plus the spread paid on entry and exit, which makes a cheap fund that trades thinly a false economy for anyone transacting often. A fund charging 0.03% with a wide spread can cost more per round trip than one charging 0.20% that trades tightly.
The index is the product
An index fund's returns are determined almost entirely by the rulebook of the index it tracks, and those rulebooks differ far more than their names suggest. Two funds both described as tracking the US market can differ in how many companies they hold, whether they weight by market capitalization or equally, and what they exclude.
Weighting is the largest of those choices. A capitalization-weighted index puts the most money into whatever has already grown the most, so a concentrated market produces a concentrated fund. That is a description of the rule rather than a criticism of it, and it is disclosed in the fund's documents.