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Doesn't the increased in implied volatility infringe the semi-strong form of the EMH? Whether you buy options 7 or 1 day before the earnings date, you still don't know any more about the actual earnings! So why would options 7 days before be less volatile than those 1 day before the earnings date?

I quote Zvi Bodie, Alex Kane, Alan J. Marcus's Investments (2018 11 edn). p 338.

      The semistrong-form hypothesis states that all publicly available information regarding the prospects of a firm must be reflected already in the stock price. Such information includes, in addition to past prices, fundamental data on the firm’s product line, quality of management, balance sheet composition, patents held, earnings forecasts, and accounting practices. Again, if investors have access to such information from publicly available sources, one would expect it to be reflected in stock prices.

This Charles Schwab article counsels that 'the best performing strategy was purchasing calls, puts, or both (long straddle) about one week before earnings, and then closing out those positions about one day before earnings, as the spike in volatility caused all of the options to gain value, despite the relative stability of the stock price.'

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The semistrong-form hypothesis states that all publicly available information regarding the prospects of a firm must be reflected already in the stock price. Such information includes, in addition to past prices, fundamental data on the firm’s product line, quality of management, balance sheet composition, patents held, earnings forecasts, and accounting practices. Again, if investors have access to such information from publicly available sources, one would expect it to be reflected in stock prices.

The information cited in the above quote is past and present information. An earnings announcement offers new information. If may be confirming current information or it may be providing new information that is relevant to future cash flow. These are unknowns.

The more certain the new is, the less likely that there will be a change in implied volatility (for example, an electric utility stock). The more uncertain the future is, the higher the implied volatility will rise as traders bid up option prices in anticipation of new information that may lead to significant price change (for example, most options in March when the market was tanking 30+ pct).

Implied volatility tends to begin increasing 2-4 weeks before an EA. Some recommend buying straddles 1-2 weeks before the EA in order to capture the increase in IV, hoping to offset the double sided time decay. This strategy also offers the possibility of capturing a large move in the underlying before the EA.

AFAIC, it's not a good strategy to own long options for an EA. The IV contraction can be severe, making it harder to break even, let alone profit. A better strategy is combo positions, selling some of that expensive IV to offset long IV.

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    This answer doesn't address the question of why an earnings outcome would be more uncertain just before the announcement than it was a week before.
    – nanoman
    May 21 '20 at 23:47
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    An earnings announcement offers new information. If may be confirming current information or it may be providing new information that is relevant to future cash flow. These are unknowns. Hence the uncertainty that leads to the increased demand for options with resultant IV increase. Feel free to provide what you believe to be a better answer. Jun 21 '20 at 2:33

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