{"format":"alysly-page/1","url":"https://alysly.com/learn/what-is-an-index/","json":"https://alysly.com/json/learn/what-is-an-index/index.json","lang":"en-US","title":"What Is an Index? Rules Instead of Picks – Alysly","description":"An index is a rules-based basket of securities — the thing an ETF tracks. How the rules work and why weights follow market value.","published":"2026-09-18T13:56:40+02:00","modified":"2026-11-01T18:26:04+01:00","alternates":[{"lang":"de-DE","url":"https://alysly.com/de/lernen/was-ist-ein-index/","json":"https://alysly.com/json/de/lernen/was-ist-ein-index/index.json"}],"about":"Machine-readable version of the page at url. It contains the text, tables and key figures published there, but not the interactive charts and tools.","breadcrumb":[{"name":"Investing & Trading Basics","url":"https://alysly.com/learn/"},{"name":"Level 0","url":"https://alysly.com/learn/#level-0"},{"name":"What Is an Index? Rules Instead of Picks","url":"https://alysly.com/learn/what-is-an-index/"}],"content":[{"type":"paragraph","text":"Level 0 Investing & ETF Basics · Part 2/16"},{"type":"heading","level":1,"text":"What Is an Index? Rules Instead of Picks"},{"type":"paragraph","text":"By Alexander Bock, founder and data analyst","links":[{"text":"Alexander Bock","href":"https://alysly.com/about/"}]},{"type":"paragraph","text":"An index is a rules-based list of securities that measures a market. The S&P 500 covers 500 large US firms, the MSCI World around 1,250 firms across developed markets. An ETF copies such an index instead of picking holdings itself.","links":[{"text":"S&P 500","href":"https://alysly.com/markets/indices/sp500/"},{"text":"MSCI World","href":"https://alysly.com/markets/indices/msci-world/"}]},{"type":"image","src":"https://alysly.com/img/ta/index-weights.33637ba84312c2a179d5bbd1d07d326a8e3543042c7ca3ebf9b39bc2d1de0646.webp","alt":"Ten bars of different heights standing for company size; the three largest together carry as much weight as the seven smaller ones"},{"type":"paragraph","text":"An index usually weights by market value: (1) the largest company contributes most to the index move, (2) the smallest barely at all. (3) The three largest bars here weigh as much as all the others combined, which is why a single heavyweight moves the whole index noticeably."},{"type":"heading","level":2,"id":"rules-decide-not-a-manager","text":"Rules Decide, Not a Manager"},{"type":"paragraph","text":"Every index has a written rulebook. It sets out which securities qualify at all (country, listing, minimum size, trading volume). It also fixes how often the list is reviewed and how weights are handed out. Membership follows from those rules."},{"type":"paragraph","text":"The most common approach is market-cap weighting: a company’s stock market value sets its share. When a price doubles, the share grows with it, and nobody intervenes."},{"type":"heading","level":2,"id":"a-live-example","text":"A Live Example"},{"type":"paragraph","text":"Live data"},{"type":"heading","level":2,"text":"What's Inside – S&P 500"},{"type":"paragraph","text":"Based on: State Street® SPDR® S&P® 500 UCITS ETF (Dist) · as of Oct 30, 2026 · 504 holdings · index page S&P 500","links":[{"text":"index page S&P 500","href":"https://alysly.com/markets/indices/sp500/"}]},{"type":"table","caption":"What's inside – S&P 500","columns":["#","Stock","Weight"],"rows":[["1","Apple Inc. AAPL","7.70%"],["2","NVIDIA Corporation NVDA","7.59%"],["3","Microsoft Corporation MSFT","5.94%"],["4","Amazon.com Inc. AMZN","3.61%"],["5","Alphabet Inc. Class A GOOGL","2.51%"],["6","Broadcom Inc. AVGO","2.38%"],["7","Meta Platforms Inc Class A META","2.08%"],["8","Alphabet Inc. Class C GOOG","2.02%"],["9","Eli Lilly LLY","1.74%"],["10","Tesla Inc. TSLA","1.72%"]]},{"type":"heading","level":3,"text":"Sectors"},{"type":"facts","items":[{"label":"IT","value":"37.4%"},{"label":"Finanzwesen","value":"12.2%"},{"label":"Gesundheitsversorgung","value":"10.3%"},{"label":"Zyklische Konsumgüter","value":"9.0%"},{"label":"Kommunikation","value":"8.4%"},{"label":"Industrie","value":"8.2%"}]},{"type":"paragraph","text":"Nobody picks the weights above. They follow the index rules and shift as prices move."},{"type":"heading","level":2,"id":"what-follows-from-it","text":"What Follows From It"},{"type":"list","ordered":false,"items":["The index decides what sits inside the fund and how concentrated it is. Under market-cap weighting a handful of large holdings carry a sizeable part of the move.",{"text":"Two ETFs on the same index are nearly identical inside. The differences are cost, income treatment and replication method — see ETF fees.","links":[{"text":"ETF fees","href":"https://alysly.com/learn/etf-fees/"}]},{"text":"The study US weight in the MSCI World measures in detail how concentrated the MSCI World really is.","links":[{"text":"US weight in the MSCI World","href":"https://alysly.com/analysis/msci-world-us-weight/"}]},{"text":"Every index we cover is listed in the indices overview.","links":[{"text":"indices overview","href":"https://alysly.com/markets/indices/"}]}]},{"type":"heading","level":2,"id":"other-weighting-methods","text":"Other Weighting Methods"},{"type":"list","ordered":false,"items":["Price-weighted: the Dow Jones Industrial Average and the Nikkei 225 weight by share price, not market value. An expensive stock counts for more than a cheap one, regardless of the company’s size.","Equal-weighted: every stock gets the same share, so large and small companies count equally.","Capped: the weights of the largest stocks are limited, as in many sector indices."]},{"type":"heading","level":2,"id":"price-index-or-total-return-index","text":"Price Index or Total Return Index"},{"type":"paragraph","text":"A price index shows only price moves; a total return index reinvests paid-out dividends. The DAX is calculated as a total return index, while the S&P 500 is usually quoted as a price index. Over long periods the gap becomes large, so the two kinds cannot be compared directly."},{"type":"paragraph","text":"Information only, not investment advice."},{"type":"list","ordered":false,"items":[{"text":"Previous What Are ETFs? Simply Explained","href":"https://alysly.com/learn/what-are-etfs/"},{"text":"Next ETFs for Beginners: The Five Basic Decisions","href":"https://alysly.com/learn/etf-for-beginners/"}]},{"type":"heading","level":2,"text":"Indices"},{"type":"table","caption":"Indices, performance over the selected period","columns":["Index","1M"],"rows":[[{"text":"Dow Jones","href":"https://alysly.com/markets/indices/dow-jones/"},{"text":"+1.7%","data":{"m1":"+1.7%","m3":"-1.4%","m6":"+4.5%","ytd":"+7.6%","max":"+30.1%"}}],[{"text":"S&P 500","href":"https://alysly.com/markets/indices/sp500/"},{"text":"-0.9%","data":{"m1":"-0.9%","m3":"+1.2%","m6":"+4.9%","ytd":"+10.8%","max":"+35.8%"}}],[{"text":"MSCI World","href":"https://alysly.com/markets/indices/msci-world/"},{"text":"-0.8%","data":{"m1":"-0.8%","m3":"+0.6%","m6":"+4.0%","ytd":"+10.2%","max":"+35.3%"}}],[{"text":"MSCI ACWI","href":"https://alysly.com/markets/indices/msci-acwi/"},{"text":"-1.7%","data":{"m1":"-1.7%","m3":"-0.2%","m6":"+2.9%","ytd":"+10.3%","max":"+41.7%"}}]]},{"type":"heading","level":2,"text":"ETFs"},{"type":"table","caption":"ETFs, performance over the selected period","columns":["Fund","1M"],"rows":[[{"text":"SPDR S&P 500 ETF Trust SPY","links":[{"text":"SPDR S&P 500 ETF Trust","href":"https://alysly.com/markets/etfs/us/spy/"}]},{"text":"-0.9%","data":{"m1":"-0.9%","m3":"+1.2%","m6":"+4.9%","ytd":"+10.8%","max":"+35.8%"}}],[{"text":"Vanguard S&P 500 ETF VOO","links":[{"text":"Vanguard S&P 500 ETF","href":"https://alysly.com/markets/etfs/us/voo/"}]},{"text":"-0.9%","data":{"m1":"-0.9%","m3":"+1.2%","m6":"+4.9%","ytd":"+10.8%","max":"+34.3%"}}],[{"text":"iShares Core MSCI World UCITS ETF USD (Acc) EUNL","links":[{"text":"iShares Core MSCI World UCITS ETF USD (Acc)","href":"https://alysly.com/markets/etfs/ishares-core-msci-world-ucits-etf-usd-acc/"}]},{"text":"-0.8%","data":{"m1":"-0.8%","m3":"+0.6%","m6":"+4.6%","ytd":"+10.9%","max":"+489.0%"}}],[{"text":"Xtrackers MSCI Emerging Markets UCITS ETF 1c XMME","links":[{"text":"Xtrackers MSCI Emerging Markets UCITS ETF 1c","href":"https://alysly.com/markets/etfs/xtrackers-msci-emerging-markets-ucits-etf-1c/"}]},{"text":"-6.3%","data":{"m1":"-6.3%","m3":"+2.4%","m6":"+0.7%","ytd":"+15.2%","max":"+95.2%"}}]]},{"type":"heading","level":2,"text":"Stocks"},{"type":"table","caption":"Stocks, performance over the selected period","columns":["Stock","1M"],"rows":[[{"text":"NVIDIA Corporation NVDA","links":[{"text":"NVIDIA Corporation","href":"https://alysly.com/markets/stocks/nvidia-corporation/"},{"text":"United States","href":"https://alysly.com/markets/stocks/browse/?country=United%20States"}]},{"text":"-10.4%","data":{"m1":"-10.4%","m3":"+1.9%","m6":"+3.1%","ytd":"+9.7%","max":"+60.6%"}}],[{"text":"Apple Inc. AAPL","links":[{"text":"Apple Inc.","href":"https://alysly.com/markets/stocks/apple-inc/"},{"text":"United States","href":"https://alysly.com/markets/stocks/browse/?country=United%20States"}]},{"text":"+3.5%","data":{"m1":"+3.5%","m3":"+11.6%","m6":"+23.0%","ytd":"+26.8%","max":"+51.4%"}}],[{"text":"Tesla Inc. TSLA","links":[{"text":"Tesla Inc.","href":"https://alysly.com/markets/stocks/tesla-inc/"},{"text":"United States","href":"https://alysly.com/markets/stocks/browse/?country=United%20States"}]},{"text":"+12.7%","data":{"m1":"+12.7%","m3":"+28.5%","m6":"+2.3%","ytd":"-11.1%","max":"+65.9%"}}]]},{"type":"paragraph","text":"Data note 1","href":"#dn-1"},{"type":"heading","level":2,"text":"More Studies"},{"type":"list","ordered":false,"items":[{"text":"Jul 21, 2026 Collectibles ×5.4 for the same cards, matched-model index, Mar 2022 – Aug 2026 Is the Pokémon Card Market in a Bubble? 4,159 Sales Analyzed Prices for PSA-graded cards multiplied. Measured on the same cards they rose 438%, while the raw median shows 786% because pricier cards came up for sale later.","href":"https://alysly.com/analysis/pokemon-card-market-bubble/"},{"text":"May 19, 2026 Crypto 45.6% Bitcoin’s annualized volatility, Jul 2024 – Sep 2026 Bitcoin in Investment Portfolios: The Real Numbers Bitcoin is calmer than it was and still about three times as volatile as the stock market. It moves with stocks, not with gold.","href":"https://alysly.com/analysis/bitcoin-investment-portfolios/"},{"text":"Mar 17, 2026 Wealth €1.2m needed for €3,000 a month indefinitely at a 3% real return Living Off Your Returns: What It Really Takes The capital you need is arithmetic. The return you assume is a guess, and it can triple the answer.","href":"https://alysly.com/analysis/living-off-returns/"}]},{"type":"paragraph","text":"View all studies","href":"https://alysly.com/analysis/#studies"},{"type":"heading","level":2,"text":"Learn"},{"type":"list","ordered":false,"items":[{"text":"ETFs for Beginners: The Five Basic Decisions Time horizon, brokerage account, choice of index, savings plan or lump sum, and how to handle market declines: the five points that shape an ETF start.","href":"https://alysly.com/learn/etf-for-beginners/"},{"text":"ETF Names Decoded: What Each Part Means iShares Core MSCI World UCITS ETF USD (Acc) – every part of an ETF name explained.","href":"https://alysly.com/learn/decode-etf-names/"}]},{"type":"paragraph","text":"Course overview","href":"https://alysly.com/learn/"},{"type":"paragraph","text":"Updated Nov 1, 2026 · Content and figures checked Nov 1, 2026"},{"type":"heading","level":2,"id":"note-legend-h","text":"Data Notes"},{"type":"paragraph","text":"All calculations run automatically and are not investment advice."},{"type":"list","ordered":true,"items":[{"id":"dn-1","text":"Returns computed automatically from the price series, excluding dividends, taxes and costs."}]},{"type":"paragraph","text":"Data and methodology: Alysly · Responsible for content: Alexander Bock","links":[{"text":"Alysly","href":"https://alysly.com/methodology/"},{"text":"Alexander Bock","href":"https://alysly.com/about/"}]},{"type":"paragraph","text":"View all data notes","href":"https://alysly.com/notes/"}],"structured_data":[{"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://alysly.com/#org","name":"Alysly","url":"https://alysly.com/","logo":{"@type":"ImageObject","url":"https://alysly.com/img/icon-512.png","width":512,"height":512},"description":"Data-driven insights, transparent analysis and powerful tools for long-term investors.","email":"mail@alysly.com","contactPoint":{"@type":"ContactPoint","contactType":"customer support","email":"mail@alysly.com","availableLanguage":["de","en"]},"address":{"@type":"PostalAddress","addressLocality":"Haltern am See","postalCode":"45721","addressCountry":"DE"},"founder":{"@id":"https://alysly.com/#founder"},"sameAs":["https://www.linkedin.com/company/alyslycom/","https://www.youtube.com/@alyslycom","https://x.com/alyslycom"],"publishingPrinciples":"https://alysly.com/methodology/"},{"@type":"Person","@id":"https://alysly.com/#founder","name":"Alexander Bock","jobTitle":"Founder and data analyst","url":"https://alysly.com/about/","sameAs":["https://www.linkedin.com/in/abxk/"]},{"@type":"WebSite","@id":"https://alysly.com/#site","url":"https://alysly.com/","name":"Alysly","alternateName":"Alysly Investment Analysis","inLanguage":"en-US","publisher":{"@id":"https://alysly.com/#org"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://alysly.com/markets/stocks/browse/?q={search_term_string}"},"query-input":"required name=search_term_string"}},{"@type":"WebPage","@id":"https://alysly.com/learn/what-is-an-index/#webpage","url":"https://alysly.com/learn/what-is-an-index/","name":"What Is an Index? 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