Learn about market efficiency (EMH) debate — Hinglish notes

6.6.10 · Stock-Market › Factor & Behavioral Finance

Overview

Efficient Market Hypothesis (EMH) finance ka central battleground hai: kya aap market ko beat kar sakte ho, ya sari public information prices mein already reflect ho chuki hai? Yeh debate investment strategy, academic research, aur hum price discovery ko kaise samajhte hain — sab ko shape karti hai.

Figure — Learn about market efficiency (EMH) debate

Core Concepts

The Three Forms of EMH

Yeh hierarchy kyun? Har form pichle se stronger (zyada restrictive) hai. Agar semi-strong hold karta hai, to weak bhi hold karna chahiye (past prices public info ka ek subset hain). Yeh hierarchy humein alag levels par efficiency test karne deti hai.

Deriving the Random Walk from First Principles

Question: Efficiency random walk kyun imply karta hai?

Derivation:

Step 1: Total return par pricing condition define karo. Investors ko price appreciation aur dividends $D_{t + 1}$ ke zariye pay kiya jaata hai. Agar market return $r$ require karti hai, to aaj ka efficient price satisfy karta hai: $P_{t} = \frac{E _{t} [ P _{t + 1} + D _{t + 1} ]}{1 + r}$ Yeh total payoff (price plus cash flow) par present value hai. $D$ kyun include karo? EMH total returns price karta hai; dividends ignore karna investors ki actual earnings ko mis-state kar dega.

Step 2: Expected total payoff ke liye rearrange karo: $E_{t} [P_{t + 1} + D_{t + 1}] = P_{t} (1 + r) ⟹ E_{t} [\frac{P _{t + 1} + D _{t + 1} - P _{t}}{P _{t}}] = r$ Expected total return required return $r$ ke barabar hota hai — koi predictable excess return exist nahi karta.

Step 3: Realized price introduce karo. Realized total payoff ko uski expectation plus ek surprise ke roop mein likhte hain: $P_{t + 1} + D_{t + 1} = P_{t} (1 + r) + ϵ_{t + 1}$ jahaan $ϵ_{t + 1}$ ek surprise hai — woh news jo time $t$ par known nahi thi. Surprise kyun? Kyunki agar yeh predictable hota, to yeh already time- $t$ expectation mein hota.

Step 4: $ϵ$ ko characterize karo. Efficiency ki definition se, $ϵ_{t + 1}$ hai:

Zero mean: $E_{t} [ϵ_{t + 1}] = 0$ (koi predictable component nahi)
Time- $t$ information se uncorrelated: $C o v (ϵ_{t + 1}, I_{t}) = 0$ kisi bhi info $I_{t}$ ke liye jo $t$ par available hai

Step 5: Random walk ke dono forms ko connect karo.

Agar required return $r = 0$ aur dividends zero hain, hum pure random walk (no drift) recover karte hain: $P_{t + 1} = P_{t} + ϵ_{t + 1}$ .
Agar $r > 0$ , prices average par upward trend karti hain: ek random walk with drift. Drift sirf time value of money aur risk ke liye compensation hai, na ki koi predictable arbitrage.

The Case For EMH: Evidence

The Case Against EMH: Anomalies

Yeh kyun matter karte hain: Agar efficiency hold karta, to yeh patterns arbitrage ho jaate. Yeh fact ki yeh persist karte hain, limits to arbitrage ya behavioral biases suggest karta hai.

Behavioral Explanations: Why Efficiency Fails

Key Behavioral Mechanisms

Overconfidence: Traders apni ability overestimate karte hain, leading to excessive trading aur hard-to-value stocks ki mispricing.
Representativeness heuristic: Investors random data mein patterns dekhte hain ("hot hand fallacy"), momentum drive karte hue.
Limits to arbitrage: Even agar koi stock mispriced hai, arbitrageurs face karte hain:
- Fundamental risk: Mispricing correct hone se pehle aur worsen ho sakta hai
- Noise trader risk: Irrational traders prices ko aur door push kar sakte hain
- Horizon constraints: Short lockup periods wale hedge funds long-term mean reversion ka wait nahi kar sakte
- Borrowing costs: Overpriced stocks ko short karna expensive ya impossible hota hai

Yeh kyun matter karta hai: Free, riskless arbitrage ke bina, mispricings persist karti hain. Yahi Limits to Arbitrage argument hai (Shleifer & Vishny, 1997).

The Synthesis: Adaptive Markets Hypothesis

Example: Momentum premium 1990s ke baad shrink hua jab zyada funds ne momentum strategies adopt karin. Yeh adaptive efficiency ke consistent hai — anomalies real hain but jaise hi woh known ho jaate hain arbitrage ho jaate hain.

Practical Implications

Agar aap believe karte ho...	To aapko...	Kyun?
Strong EMH	100% passive indexing	Stocks pick karne ya market time karne ki koshish ka koi faida nahi
Weak anomalies exist	Factor investing (value, momentum, quality ki taraf tilt)	Behavioral biases ki wajah se persist karne wale premiums capture karo
Markets are exploitable	Active management + short-term trading	Dusron se pehle mispricings identify karne ki koshish karo
Adaptive efficiency	Hybrid approach: passive core + tactical tilts	Yeh recognize karo ki alpha rare hai aur decay karta hai, but impossible nahi

Common Mistakes & Steel-manning

Connections

Factor Investing Basics: Value, momentum, aur quality factors semi-strong inefficiencies exploit karte hain
Behavioral Biases in Trading: Overconfidence, herding, aur loss aversion anomalies drive karte hain
Limits to Arbitrage: Known hone par bhi mispricings kyun persist karti hain
Index Funds vs Active Management: EMH ka main practical consequence
Random Walk Theory: Weak-form efficiency ki mathematical foundation
Fama-French Three-Factor Model: Market, size, value — risk-based benchmark
Carhart Four-Factor Model: Three-factor model mein momentum add karta hai

Recall Feynman: Explain to a 12-Year-Old

Socho tum Pokémon cards trade kar rahe ho. Efficient Market Hypothesis kehta hai: "Har card ki price already fair hai, kyunki hazaron bachche constantly prices check kar rahe hain aur best deal paane ke liye trade kar rahe hain."

Agar Charizard $100 k ahaia u r t u mh ee k$ 80 mein milta hai, tum turant khareed lete ho. But 50 aur bachche bhi aisa karte hain. Seconds ke andar, seller demand ki wajah se price $100 kar deta hai. Tum "deal" nahi pa sakte kyunki bheed bahut smart aur fast hai.

But yahan twist hai: kabhi kabhi zyaadatar bachche believe karte hain ki Charizard agale saal super rare hoga, isliye sab khareedne lagte hain, price $120 t ak p u s hk a r d e t e hain e v e n t h o ug h " s a c hm e in "$ 100 worth hai. Yeh ek behavioral bias hai — sab saath mein excited hain. Ya shayad kuch bachche notice karte hain ki jo cards pichle mahine upar gaye woh is mahine bhi upar jaate rehte hain (momentum), to woh trend ride karte hain.

EMH debate yeh hai: "Kya bachche itne smart hain ki prices hamesha fair hain?" vs. "Kya bachche predictable emotional mistakes karte hain?" Sach kahin beech mein hai. Prices zyaadatar time mein mostly fair hain, but agar tum clever ho (ya lucky), shayad tum mistakes spot kar lo.

Active Recall Flashcards

#flashcards/stock-market

Efficient Market Hypothesis (EMH) asset prices ke baare mein kya claim karta hai?

Prices sari available information fully reflect karti hain, jisse us information ka use karke consistently market beat karna impossible ho jaata hai.

EMH ka weak form kya hai?

Prices saare past price aur volume data reflect karti hain, isliye technical analysis excess returns generate nahi kar sakta.

EMH ka semi-strong form kya hai?

Prices saari publicly available information (news, earnings, reports) reflect karti hain, isliye fundamental analysis excess returns generate nahi kar sakta.

Random walk with aur without drift mein kya farak hai?

Without drift:

P_{t + 1} = P_{t} + ϵ_{t + 1}

(fair game,

r = 0

, no dividends). With drift:

P_{t + 1} + D_{t + 1} = P_{t} (1 + r) + ϵ_{t + 1}

, jahaan upward trend sirf time value aur risk ke liye compensation hai, koi predictable arbitrage nahi.

EMH pricing mein sirf price changes ki jagah total returns kyun use karne chahiye?

Kyunki investors price appreciation AUR dividends dono se earn karte hain; dividends ignore karna actual return ko mis-state karta hai, isliye pricing condition

E_{t} [P_{t + 1} + D_{t + 1}] / (1 + r)

use karta hai.

Do anomalies batao jo EMH ko challenge karte hain.

(1) Momentum: past winners outperform karte rehte hain; (2) Post-earnings announcement drift: earnings surprises ke baad prices weeks tak slowly adjust hoti hain.

Fama-French three-factor model mein kaun se factors hain, aur kaun sa model momentum add karta hai?

Fama-French (1992) = market, size (SMB), value (HML). Carhart (1997) momentum ko fourth factor ke roop mein add karta hai.

"Limits to arbitrage" argument kya hai?

Even agar mispricings exist karein, arbitrageurs fundamental risk, noise trader risk, aur borrowing costs face karte hain, isliye woh anomalies poori tarah eliminate nahi kar sakte.

Adaptive Markets Hypothesis kya claim karta hai?

Market efficiency dynamic aur context-dependent hai; anomalies exist karti hain but known hone par arbitrage ho jaati hain, aur efficiency time aur assets mein vary karti hai.

EMH kyun imply karta hai ki indexing optimal hai?

Agar prices information fully reflect karti hain, to active management systematically market beat nahi kar sakta, isliye low-cost passive indexing market returns capture karta hai bina fees waste kiye.

Overconfidence bias kya hai aur yeh EMH ko kaise violate karta hai?

Investors winners pick karne ki apni ability overestimate karte hain, leading to excessive trading aur hard-to-value stocks ki mispricing, jo fully efficient markets mein hona nahi chahiye.

January effect kya hai?

Small-cap stocks ne historically January mein abnormally high returns earn kiye hain, ek seasonal anomaly jo weak-form efficiency se inconsistent hai.