Pro Investor Academy
LESSON 3.2
6 min learning time

Index Investing

An index fund tries to match an index rather than beat it.

The short answer

An index fund holds the constituents of a chosen index in roughly the index's proportions, so its return closely follows that index rather than a manager's individual selections. This passive approach keeps costs low, and independent long-run research finds that low-cost index exposure has historically outperformed most active alternatives after fees.

Learning objectives

  • Explain the difference between an index and an index fund
  • Describe how an index decides which companies to include and how much weight each gets
  • State why low costs are structural to passive investing, not incidental
  • Interpret tracking difference as a measure of fund quality

An index is a measurement, not a fund

An index such as the S&P 500 is a rulebook: it defines which companies qualify, how often the list is reviewed, and how much weight each company gets. The index itself cannot be bought — it is simply a number calculated from the prices of its constituents.

An index fund is a real, investable product built to hold those same constituents in roughly the same proportions (or a representative sample), so its return tracks the index as closely as possible after costs.

A fund's net asset value (NAV) is its assets minus its liabilities; divided by shares outstanding it gives NAV per share. Even an index fund trades for reasons other than index changes — to invest new money, pay out redemptions, reinvest dividends and handle corporate actions such as mergers and spin-offs.

Tracking instead of picking

An index fund holds the constituents of an index in roughly the index's proportions. No one is choosing which companies look attractive, so costs are low.

Active funds attempt to beat an index through selection. Independent research from S&P Dow Jones Indices (the SPIVA Scorecards) has repeatedly found that most active funds underperform their benchmark over long periods, largely because of costs.

How weighting works

Most well-known indexes, including the S&P 500, are market-capitalization weighted: larger companies make up a larger share of the index. This means an index fund's return is disproportionately influenced by the largest handful of companies in it, even though it may hold hundreds of names.

What tracking means in practice

A good index fund is judged by how closely it tracks its index after costs, not by whether it went up. Tracking difference and expense ratio are the numbers that matter, because the index's return is a given — the fund's job is only to deliver as much of it as possible.

Educational example · Fund

Vanguard S&P 500 ETF (VOO)

This is a well-known example of a fund built to track the S&P 500 index rather than to select individual companies. It illustrates the structure discussed in this lesson: broad, market-capitalization-weighted exposure at a low ongoing cost. Nothing here is a recommendation; review the fund's own materials and holdings before drawing conclusions.

Read the fund sponsor's official page

Named to illustrate the concept only. This is not a recommendation to buy or sell any investment.

Key concepts

Select a term to read its definition.

See the full glossary

Why this matters

Low-cost, broad ownership is the baseline every other strategy should be measured against.

Common mistakes

  • Judging an index fund by its recent return instead of its tracking difference and cost.

Check your understanding

No score is recorded. This is only here to test whether the lesson landed.

1. What is the relationship between an index and an index fund?
2. True or false: in a market-capitalization-weighted index, every constituent company has an equal influence on returns.
3. A hypothetical fund's return closely matches its stated index every year after fees. What does that consistency tell you about the fund?

0 of 3 answered. Answer every question to see the answers.

Key takeaways

  • An index is a rulebook and a number; an index fund is the investable product built to track it
  • Most broad indexes are weighted by market capitalization, concentrating influence in the largest companies
  • Low cost is structural to passive investing because no one is being paid to select holdings
  • Independent long-run research finds most active managers underperform their benchmark after fees

Optional next reading

Not required for this level or the certificate.

Related concepts

Written by
Pro Investor Academy Research Team
Content type
Educational article
Published
2026-09-13
Last reviewed
2026-09-13

Sources

What Is an ETF?Costs and Expense Ratios

Educational content only. This lesson is not investment, tax or legal advice and does not recommend buying or selling any specific investment. All investing involves the risk of loss.