Program trading is usually either one of two things...
1. Arbitrage trading between related indexes (SPX futures vs. SPX cash).
2. Computerized buy and sell programs based on proprietary trading systems.
One thing which has possibly contributed to the volume in the first instance is the decimalization of stock quotes, which in turn has enhanced volume and liquidity in general making it easier to do big basket arbitrage transactions.
Something that could be contributing to the second type, is an increase in the number of buy and sell signals due to the fact that we have been in a whipsaw trading range for the last six months.
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