as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
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judge market efficiency from raw price action
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the shortest-hand yet most reliable metric is daily volume / total shares outstanding.  a security with a high turnover rate will be more efficient than a lower one, ceteris paribus. the practical impacts are tighter spread and lower average percentage change between trades.  a security with a spread of 0% and an average change of 0% between trades is perfectly efficient.
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