grad-emh
用效率市場假說評估價格有沒有反映資訊，策略能不能賺到異常報酬
中文翻譯
--- name: "grad-emh" description: "用效率市場假說（Fama，1970）從弱式、半強式、強式三種形式，評估資產價格對資訊的吸收程度。當使用者需要評估市場效率、判斷某個交易策略能不能產生異常報酬、評估事件研究，或問到「技術分析有用嗎」「市場是不是已經知道這件事了」「這個異常現象能拿來賺錢嗎」時，就用這個 skill。" metadata: category: "WP-26 財務理論" tags: ["EMH", "efficient-market", "Fama", "market-efficiency", "information", "anomalies"] --- # Efficient Market Hypothesis (EMH) ## Overview 效率市場假說（Fama，1970）主張資產價格已經充分反映可得資訊，所以不可能持續賺到異常報酬。EMH 分成三種形式，也就是弱式、半強式和強式，各自由價格所反映的資訊集合來定義。 ## When to Use - 評估某個交易策略是不是真的利用了無效率 - 設計事件研究（半強式的檢定） - 評估主動管理相對被動指數化有沒有增加價值 - 討論技術分析或基本面分析是否有效 ## When NOT to Use - 拿來當藉口，不加調查就忽略所有市場異常現象 - 市場明顯缺乏流動性，或資訊被分割 - 做規範性的主張：EMH 描述的是價格行為，不是價格「應該」是多少 ## Assumptions ``` IRON LAW: In an efficient market, prices reflect available information — beating the market consistently requires either superior information or accepting more risk. No free lunch. ``` 關鍵假設： 1. 有大量理性、追求利潤最大化的參與者 2. 資訊沒有成本，而且同時提供給所有參與者 3. 交易成本不會阻礙根據資訊進行交易 4. 投資人對新資訊反應快速而且沒有偏誤 ## Methodology ### Step 1 — Identify the Information Set - 弱式：只有過去的價格和成交量 - 半強式：所有公開可得的資訊 - 強式：所有資訊，包括私有或內線資訊 ### Step 2 — Determine the Testable Implication | Form | Information Reflected | Implication | |------|----------------------|-------------| | Weak | 歷史價格 | 技術分析無法賺到超額報酬 | | Semi-strong | 所有公開資訊 | 基本面分析無法賺到超額報酬 | | Strong | 所有資訊（公開加私有） | 連內線人士也無法賺到超額報酬 | ### Step 3 — Select Appropriate Test - 弱式：自相關檢定、連串檢定（runs test）、濾嘴法則 - 半強式：事件研究（公告前後的異常報酬） - 強式：內線交易獲利研究 ### Step 4 — Interpret Results with Joint-Hypothesis Awareness 任何效率檢定，同時也是在檢定用來定義「異常」報酬的資產定價模型。 ## Output Format ```markdown ## EMH Assessment: [Market / Strategy] ### Efficiency Form Tested - Form: [weak / semi-strong / strong] - Information set: [description] ### Evidence | Test | Result | Supports Efficiency? | |------|--------|---------------------| | [test name] | [finding] | [Yes/No/Ambiguous] | ### Known Anomalies in This Context - [List relevant anomalies and their current status] ### Conclusion - [Efficiency assessment with caveats] - [Joint-hypothesis caveat] ``` ## Gotchas - 聯合假設問題：不假設一個均衡模型，就沒辦法檢定效率 - Grossman-Stiglitz 悖論（1980）：如果市場完全有效率，就沒有人有動機去蒐集資訊 - 異常現象（動能、價值、規模）持續存在，但可能反映風險或資料挖掘 - EMH 並沒有主張價格永遠「正確」，只是主張錯誤定價無法被系統性地利用 - 市場效率因市場區塊而異，大型股比微型股更有效率 - 行為財務學提供了系統性的反例，但不一定推翻 EMH ## References - Fama, E. (1970). Efficient capital markets: a review of theory and empirical work. *Journal of Finance*, 25(2), 383-417. - Grossman, S. & Stiglitz, J. (1980). On the impossibility of informationally efficient markets. *American Economic Review*, 70(3), 393-408. - Malkiel, B. (2003). The efficient market hypothesis and its critics. *Journal of Economic Perspectives*, 17(1), 59-82.
英文原文
--- name: "grad-emh" description: "Apply the Efficient Market Hypothesis (Fama, 1970) to evaluate information incorporation in asset prices across weak, semi-strong, and strong forms. Use this skill when the user needs to assess market efficiency, determine if a trading strategy can generate abnormal returns, evaluate event studies, or when they ask 'can technical analysis work', 'does the market already know this', or 'is this anomaly exploitable'." metadata: category: "WP-26 財務理論" tags: ["EMH", "efficient-market", "Fama", "market-efficiency", "information", "anomalies"] --- # Efficient Market Hypothesis (EMH) ## Overview The Efficient Market Hypothesis (Fama, 1970) posits that asset prices fully reflect available information, making it impossible to consistently earn abnormal returns. EMH is organized into three forms — weak, semi-strong, and strong — each defined by the information set reflected in prices. ## When to Use - Evaluating whether a trading strategy exploits genuine inefficiency - Designing event studies (semi-strong form test) - Assessing if active management adds value over passive indexing - Debating the validity of technical or fundamental analysis ## When NOT to Use - As justification to ignore all market anomalies without investigation - When markets are clearly illiquid or informationally segmented - For normative claims — EMH describes price behavior, not what prices "should" be ## Assumptions ``` IRON LAW: In an efficient market, prices reflect available information — beating the market consistently requires either superior information or accepting more risk. No free lunch. ``` Key assumptions: 1. Large number of rational, profit-maximizing participants 2. Information is costless and available simultaneously to all participants 3. Transaction costs do not prevent trading on information 4. Investors react quickly and unbiasedly to new information ## Methodology ### Step 1 — Identify the Information Set - Weak form: past prices and trading volume only - Semi-strong form: all publicly available information - Strong form: all information including private/insider information ### Step 2 — Determine the Testable Implication | Form | Information Reflected | Implication | |------|----------------------|-------------| | Weak | Historical prices | Technical analysis cannot earn excess returns | | Semi-strong | All public info | Fundamental analysis cannot earn excess returns | | Strong | All info (public + private) | Even insiders cannot earn excess returns | ### Step 3 — Select Appropriate Test - Weak: autocorrelation tests, runs tests, filter rules - Semi-strong: event studies (abnormal returns around announcements) - Strong: insider trading profitability studies ### Step 4 — Interpret Results with Joint-Hypothesis Awareness Any test of efficiency is simultaneously a test of the asset pricing model used to define "abnormal" return. ## Output Format ```markdown ## EMH Assessment: [Market / Strategy] ### Efficiency Form Tested - Form: [weak / semi-strong / strong] - Information set: [description] ### Evidence | Test | Result | Supports Efficiency? | |------|--------|---------------------| | [test name] | [finding] | [Yes/No/Ambiguous] | ### Known Anomalies in This Context - [List relevant anomalies and their current status] ### Conclusion - [Efficiency assessment with caveats] - [Joint-hypothesis caveat] ``` ## Gotchas - Joint-hypothesis problem: you cannot test efficiency without assuming an equilibrium model - Grossman-Stiglitz paradox (1980): if markets are perfectly efficient, no one has incentive to gather information - Anomalies (momentum, value, size) persist but may reflect risk or data mining - EMH does not claim prices are always "correct" — only that mispricings are not systematically exploitable - Market efficiency varies by market segment; large-cap equities are more efficient than micro-caps - Behavioral finance provides systematic counterexamples but does not necessarily invalidate EMH ## References - Fama, E. (1970). Efficient capital markets: a review of theory and empirical work. *Journal of Finance*, 25(2), 383-417. - Grossman, S. & Stiglitz, J. (1980). On the impossibility of informationally efficient markets. *American Economic Review*, 70(3), 393-408. - Malkiel, B. (2003). The efficient market hypothesis and its critics. *Journal of Economic Perspectives*, 17(1), 59-82.