Resumen de Measurement of insider trading in wagering markets
In an influential article, Shin (1993) proposed a method to calculate the extent of insider trading in bookmaker markets, with z as the measure of insider trading. He assumed that bookmakers manipulate the supply-side of the market to protect themselves against the risks of adverse selection involving counterparties with proprietary or inside information and against excessive payouts from wins by high odds horses. This article uses a large sample of thoroughbred races (n = 1796) over 4 years on Saturdays at major venues in Melbourne to validate the Shin methodology. Analysis derives a z-measure of insider trading in bookmaker markets of just over 2% (which closely matches results from multiple UK analyses). The surprise is that an almost identical value (p < 0.001) is obtained for z in the Tote market which does not have a supply side and so should have a zero value of z. It seems that factors driving a nonzero value of z arise in the demand side of wagering markets and not the supply side as assumed. This conclusion illustrates the risks associated with what Fama (1991, p. 1575) termed 'the joint hypothesis problem' where the conclusions of a mis-specified market model are likely to be invalid.
