Zmiana poziomu efektywności amerykańskiego rynku akcji – od kryzysu finansowego 2007-2009 do pandemii COVID-19
Change in the efficiency level of the us stock mark et – from the 2007-2009 financial crisis to the COVID-19 pandemic

Summary/Abstract: Purpose – Verification of the change in the efficiency of the US stock market from the 2007-2009 financial crisis to the COVID-19 pandemic. Research method – The BDS test (occurrence of non-linear relationships between daily logarithmic rates of return) and the Quenouille autocorrelation test up to the fifth order (occurrence of linear relationships between daily logarithmic rates of return) for the S&P500 and DJIA indices were used. Results – The occurrence of unusual phenomena changes the level of US stock market effecti-veness. An increase in inefficiency was observed both during the 2007-2009 financial crisis and now, during the COVID-19 pandemic. Higher-level relations ips between rates of return were found during the COVID-19 pandemic. Originality / value / implications / recommendations – A comparison of the impact of two recent global events on the level of efficiency of the US stock market. Additionally, the article indicates which of the two key US stock indices provided more frequent investment opportunities.

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