Note that the key assumption for why losses are unlikely is that they make many independent trades that are each likely to be profitable. But in real markets, outside events can suddenly make many trades all fail at the same time. This is the same reason AAA tranches of CDOs got those high ratings -- you only lose money if many obligations fail at once, but that is extremely unlikely if you think they have low correl…
Not exactly, a firm like Virtu makes money due to volume and volatility. If the market goes up, they make money, if it goes down, they make money. They don't make money when they break things, or when the volume (and volatility) is low. Source: Worked at Madison Tyler / Virtu for over 4 years, but left before their IPO.
Agree with you that HFT performance is not correlated with market direction. But given the volume of trades it would imply that many of the trades are occurring on a smaller pool of equity instruments. I'd think that given liquidity constraints and competition that there's a sweet spot in terms of number of stocks that a given strategy is actually efficient on.
So not entirely sure why we consider every trade to be iid - rapid shocks (flash crash), equity specific news etc would affect a number of trades at once changing the 51% probability?
Also does anyone know if the profitability numbers include payments to be a market-maker?