A finance and accounting concept defining a method, measure, or process used to record activity and support financial decisions. It specifies how value, risk, or performance is measured or controlled through standardized rules and routines. It does not ensure correctness without reliable inputs, appropriate assumptions, and effective review and controls. It materially affects decisions and compliance by shaping how organizations allocate capital, report results, and manage exposure. The concept is generally stable, though standards, regulation, and tools evolve over time.

Principle

Prices adjust rapidly and unbiasedly to new information; hence, publicly available information is already incorporated into current market prices.

Demonstration

If a company unexpectedly reports higher earnings, its stock price jumps immediately to reflect the news; a trader using only that public earnings release cannot reliably earn persistent abnormal profits by trading on it after the first market reaction.

Misapplication

Assuming markets are perfectly efficient in all timeframes and for every security leads to underestimating opportunities from information asymmetries, frictions, or behavioral biases; using it to dismiss any active analysis is a misuse.

Consequence

Under the hypothesis, passive investment strategies (e.g., broad-market index funds) are rational because attempts to outperform the market systematically after costs tend to fail.

Reversal

The inverse concept is that markets are inefficient and prices systematically deviate from fair value in predictable ways, implying exploitable strategies and persistent alpha.

Boundary

Applies to publicly available information and securities traded in reasonably liquid markets; it does not assert that small inefficiencies never exist, nor does it fully describe private-information-driven or illiquid markets.

Semantic Tension

Tension exists between the idealized assertion of full informational reflection and empirical findings of anomalies, transaction costs, and limits to arbitrage that produce persistent deviations.

Synthesis

The Efficient Market Hypothesis is an organizing claim about information incorporation: it frames expectations about whether publicly available data can consistently produce excess returns, while acknowledging that frictions, private information, and behavioral factors can create measurable departures.