Efektywność informacyjna polskiego rynku kapitałowego w formie słabej na przykładzie spółek z indeksu WIG20
Informational efficiency of the Polish capital market in a weak form, based on the example of companies from the WIG20 index

Summary/Abstract: Background: The article fits into the research on the informational efficiency of capital markets. This topic is often discussed in the literature on the subject, but there is no consensus among researchers as to its validity. Thanks to the use of a non-standard research method, our work provides new arguments in the discussion on the efficiency of the Polish stock market. Research purpose: The aim of the article is to verify the hypothesis of information efficiency in a weak form for the Polish equity market, represented by the WIG20 index, in the period from the beginning of the market operation till the end of 2018. Additionally, the article answers the question of how market efficiency changed over time and whether it has grown over the years along with the development of the stock market. Methods: The study was carried out using the auto-correlation test of daily rates of return, examining the efficiency in the weak form. Our approach differs from previous research in terms of the form of testing. The efficiency was checked by us independently on many two-year sub-periods of the analyzed set, and not in points for the entire, long-term period. This made it possible to identify and analyze periods of efficiency that would not be noticeable in the study on aggregated data. To the best of our knowledge, the last survey in this form was conducted for the years between 1994 and 1999. Conclusions: We believe that when studying a long period, although we can unequivocally identify the existence of efficiency or its lack, it does not fully reflect the actual situation. Our research has shown that the Polish stock market is sometimes efficiency from time to time, and these periods last up to two years and occur throughout nearly 40% of the analyzed period. Moreover, efficiency has decreased in recent years, i.e., after 2009, which suggests that institutional development is not a sufficient factor to achieve information efficiency, even to a weak degree. In recent years, the market has ceased to be efficiency more and more often and for longer periods. This means that it is possible to periodically obtain abnormal rate of return based on information about past performance.

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