Learn about S&P 500, Dow Jones, NASDAQ Composite — Stock-Market notes

Overview

Stock market indices are measurement tools that track the performance of a specific group of stocks to give us a single number representing "how the market is doing." Think of them as thermometers for market health—instead of measuring temperature, they measure the collective value of selected companies.

Each index uses different selection criteria (which stocks to include) and calculation methods (how to combine their prices into one number). The three giants—S&P 500, Dow Jones, and NASDAQ Composite—each tell a different story about the market.

The S&P 500 (Standard & Poor's 500)

What It Measures

The S&P 500 represents approximately 80% of the total US stock market value. It includes companies from all 11 major sectors (Technology, Healthcare Financials, Consumer Discretionary, etc.).

Selection Criteria:

Must be a US company
Market cap≥ $14.5 billion (as of 2026)
Adequate liquidity (average trading volume)
At least 50% of shares available to public (free float)
Positive earnings in most recent quarter AND sum of last 4 quarters

How It's Calculated

$\text{Index Level} = \frac{\sum_{i=1}^{500} P_i \times Q_i}{D} \times B$

Where:

$P_i$ = Price of stock $i$
$Q_i$ = Number of shares outstanding for stock $i$ (adjusted for free float)
$D$ Divisor (proprietary number adjusted for corporate actions)
$B$ = Base value (typically set to make the index start at a round number)

WHY market-cap weighted? A $1 change in Apple (market cap$ 3trillion) should impact the index MORE than a $1 change in a small company (market cap$ 15 billion). This weighting reflects economic reality—bigger companies have bigger influence.

Derivation from First Principles:

Goal: Create a single number representing total value of 500 companies
Total Market Value: $\text{Total} = \sum_{i=1}^{500} (\text{Price}_i \times \text{Shares}_i)$
Problem: This number would be huge (trillions). We need something human-readable.
Solution: Divide by a constant (divisor $D$ ) to scale it down
Problem #2: Corporate actions (stock splits, dividends) artificially change this number
Solution: Adjust divisor $D$ whenever a corporate action occurs to maintain continuity

Example: How the divisor adjustment works

Say the index is at 5,000 and total market cap is $40 trillion: $5000 = \frac{40,000,000,000,000}{D} \implies D = 8,000,000$

Company X (part of the500) does a 2-for-1 stock split. Its price halves, shares double, but economic value unchanged. However, the formula would now give: $\text{New calc} = \frac{39,999,900,000}{8,000,000} = 4,999.99...$

The index dropped! But nothing economically changed. So we adjust the divisor: $5000 = \frac{39,999,999,900,000}{D_{\text{new}}} \implies D_{\text{new}} = 7,999,999,980$

This keeps the index at 5,000 despite the split.

Step 1: Calculate Apple's weight in the index $w_{\text{Apple}} = \frac{3,000,000,000,000}{40,000,000,000} = 0.075 = 7.5\%$

Why this step? Market-cap weighting means each company contributes proportionally to its size.

Step 2: Calculate contribution to index movement $\Delta \text{Index} = w_{\text{Apple}} \times \Delta P_{\text{Apple}} = 0.075 \times 2\% = 0.15\%$

Why this step? A company's price change multiplied by its weight gives its isolated impact.

Answer: The S&P 500 would rise approximately 0.15% from Apple's movement alone.

The Dow Jones Industrial Average (DJIA)

What It Measures

The Dow represents30 of the most significant US companies, hand-picked by editors of the Wall Street Journal. Examples: Apple, Microsoft, Boeing, Coca-Cola, Goldman Sachs.

Key Difference from S&P 500: Only 30 stocks (vs 500), and weighted by stock price not market cap.

How It's Calculated

Where:

$P_i$ = Price of stock $i$
$D_{\text{Dow}}$ = Dow Divisor (adjusted for splits/changes, currently ~0.152 as of 2026)

WHY price-weighted? Historical accident. When Charles Dow created it in 1896, calculating market cap for 30 companies was computationally hard. Adding up prices was simple.

Derivation:

Original 1896 method: Simply average the prices $\text{Average} = \frac{P_1 + P_2 + ... + P_{30}}{30}$
Problem: When a stock splits (say, 2-for-1), its price halves, making the average drop even though no value was lost
Solution: Replace the denominator (30) with an adjustable divisor $D$ $\text{DJIA} = \frac{\sum P_i}{D}$
Maintenance: When corporate actions occur, adjust $D$ to keep the index continuous (same principle as S&P 500)

If each rises $3 (1% move for A, 10% move for B), what's the impact on the Dow?

Impact from A: $\frac{3}{D}$
Impact from B: $\frac{3}{D}$

Same impact! Even though Company B is4× larger economically and moved 10× more in percentage terms.

Why this step matters: This reveals the DJIA's quirk—a high-priced stock has more influence regardless of company size. UnitedHealth (stock price ~ $500) moves the Dow more than Apple (stock price ~$ 190) even though Apple is far more valuable.

The steel-man: Tradition and continuity have value for historical comparisons.

Why it's misleading:

Only 30 companies (vs S&P's 500) = much less representative
Price-weighting is economically arbitrary—a stock split changes influence
The S&P 500 is the index professionals actually use for benchmarking

The fix: Use the Dow for historical context and headline sentiment, but rely on the S&P 500 for actual market measurement.

NASDAQ Composite

What It Measures

The NASDAQ Composite is tech-heavy because the NASDAQ exchange historically attracted technology and growth companies (Microsoft, Apple, Amazon, Google, Tesla NVIDIA all list here). It represents about3,000-,500 companies, though the top 10 companies make up ~45% of the index due to market-cap weighting.

Selection Criteria:

Must be listed on the NASDAQ exchange
Common stocks, REITs, ADRs included
No other criteria—if you're on NASDAQ, you're in the index

How It's Calculated

Where $N$ ≈ 3,000+ (all NASDAQ-listed stocks). The calculation method is identical to the S&P 500 (market-cap weighted), but the universe is "everything on this exchange" rather than "selected large companies."

WHY include all stocks? The NASDAQ wanted comprehensive measure of their exchange's performance. The S&P 500 and Dow cherry-pick; NASDAQ Composite is all-inclusive.

Interpretation:

The broad market (S&P 500) rose moderately
Blue-chip industrials (Dow) rose slower—older economy companies lagged
Tech-heavy stocks (NASDAQ) surged—growth/tech sector had a strong day

Why this matters: Different indices tell different stories. A "market rally" might really be a "tech rally" if NASDAQ outpaces S&P 500 significantly.

Comparing the Three Indices

Feature	S&P 500	Dow Jones	NASDAQ Composite
Number of Stocks	500	30	~3,000+
Weighting Method	Market-cap	Price	Market-cap
Coverage	Large US companies	30 blue-chips	All NASDAQ stocks
Sector Bias	Balanced across 11 sectors	Slight industrial tilt	Heavy tech (45%+)
Creation Year	1957	1896	1971
Use Case	Professional benchmark	Media headline	Tech sector proxy

Why Indices Matter for Investors

1. Benchmarking Performance

If your portfolio returned +12% last year but the S&P 500 returned +18%, you underperformed the market. Indices provide the comparison baseline.

2. Passive Investing

Index funds and ETFs (Exchange-Traded Funds) replicate indices. When you buy an S&P 500 index fund, you own a tiny slice of all 500 companies automatically.

Math of index funds:

Why? The fund buys stocks in the exact weights as the index, so it moves with the index (minus management fees, typically0.03-0.20% annually).

3. Market Sentiment Indicator

A rising S&P 500 suggests investor confidence; falling suggests fear. Indices aggregate thousands of investment decisions into one signal.

Common Misconceptions

Why it's wrong: Index levels are dimensionless ratios. The Dow at 40,000 means the current value is40,000× the (adjusted) base value from 1896. It's not dollars, shares, or companies—it's a pure comparison number.

The fix: Think of indices like thermometers: 98.6°F doesn't mean "98.6 things," it's a scale-based measurement.

Why it's partially wrong: The NASDAQ Composite includes retail (Costco), healthcare (Gilead), and

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho beta, sabse pehle intuition samajhte hain. Stock market index basically ek thermometer ki tarah hota hai — jaise thermometer temperature batata hai, waise hi index market ki "sehat" batata hai ek single number mein. Ab socho ki agar tumhe poochna ho "aaj US economy kaisi chal rahi hai?", toh tum 5000+ companies ko alag-alag check nahi kar sakte, na? Isiliye S&P 500, Dow Jones, aur NASDAQ jaise indices bane — yeh kuch representative companies ka movement summarize karke ek number de dete hain. Har index ke apne selection criteria hote hain (kaunsi companies lena) aur calculation method hota hai (unke prices ko kaise combine karna).

Ab core cheez jo yahaan samajhni hai woh hai market-cap weighting. S&P 500 mein har company ka influence uske size ke hisaab se hota hai. Matlab Apple jaisi badi company (market cap $3 trillion) ka$ 1 ka change chhoti company ke $1 change se zyada impact daalega index par. Yeh economic reality ko reflect karta hai — badi companies ka asli mein market par bada asar hota hai. Formula bhi isi soch se banta hai: sab companies ka price × shares add karo, phir ek divisor (D) se divide karke number ko human-readable bana do. Aur jab koi corporate action hota hai jaise stock split, toh divisor ko adjust karte hain taaki index artificially na gire — kyunki economically toh kuch badla hi nahi.

Yeh kyun matter karta hai? Kyunki jab tum "market up 1.5%" sunte ho, toh actually yeh weighted average movement hai jahaan badi companies ka zyada bolbala hota hai. Jaise example mein dekha — Apple ka weight 7.5% tha, toh Apple 2% badhne se poora index sirf 0.15% badha. Yeh samajhna zaroori hai kyunki investing karte waqt tumhe pata hona chahiye ki index ka number kahaan se aa raha hai aur konsi companies usko drive kar rahi hain. Yeh knowledge tumhe smart investor banata hai, blindly numbers follow karne wala nahi.

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