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Monday, May 10, 2010

Should "Erroneous" Algorithmic Trades be Canceled?

Rajiv Sethi doesn't understand the logic behind the decision by Nasdaq and the New York Stock Exchange to cancel some trades that were made during last week's plunge in the stock market:

Algorithmic Trading and Price Volatility, by Rajiv Sethi: Yesterday's dramatic decline and rapid recovery in stock prices may have been triggered by an erroneous trade, but could not have occurred on this scale if it were not for the increasingly widespread use of high frequency algorithmic trading.

Algorithmic trading can be based on a variety of different strategies but they all share one common feature: by using market data as an input, they seek to exploit failures of (weak form) market efficiency. Such strategies are necessarily technical and, for reasons discussed in an earlier post, are most effective when they are rare. But they have become increasingly common recently, and now account for three-fifths of total volume in US equities... This is a recipe for disaster:

[In] a market dominated by technical analysis, changes in prices and other market data will be less reliable indicators of changes in information regarding underlying asset values. The possibility then arises of market instability, as individuals respond to price changes as if they were informative when in fact they arise from mutually amplifying responses to noise. 

Under such conditions, algorithmic strategies can suffer heavy losses. They do so not because of "computer error" but because of the faithful execution of programs that are responding mechanically to market data. The decision by Nasdaq to "cancel trades of 286 securities that fell or rose more than 60 percent from their prices at 2:40 p.m." might therefore be a mistake: it protects such strategies from their own flaws and allows them to proliferate further. Canceling trades can be justified in response to genuine human or machine error, but not in response to the implementation of flawed algorithms.

I don't know how the losses and gains from yesterday's turmoil were distributed among algorithmic traders and other market participants, but it is conceivable that part of the bounce back was driven by individuals who were alert to fundamental values and recognized a buying opportunity. ...

I would be very interested to know whether the transfer of wealth that took place yesterday as prices plunged and then recovered resulted in major losses or gains for the funds using algorithmic trading strategies. I expect that those engaged in cross-market or spot-futures arbitrage would have profited handsomely, at the expense of those relying on some form of momentum based strategies. If so, then the cancellation of trades will simply set the stage for a recurrence of these events sooner rather than later. ...

I agree - I don't understand the logic behind the decision to cancel the trades either. However, Donald Marron says there's merit to both sides of the argument, and attempts to find a compromise:

Advice to Nasdaq and the NYSE: Cancel Only 90% of the “Erroneous” Trades, by Donald Marron: Nasdaq and the New York Stock Exchange have both announced that they will cancel many trades made during the temporary market meltdown between 2:40 and 3:00 last Thursday afternoon (see, for example, this story from Reuters). These “erroneous” trades include any that were executed at a price more than 60% away from their last trade as of 2:40.

The motivation for these cancellations is clear: a sudden absence of liquidity meant that many stocks (and exchange-traded funds) temporarily traded at anomalous prices that no rational investor would have accepted.

As several analysts have noted, however, canceling these trades creates perverse incentives. It rewards the careless and stupid, while penalizing the careful and smart. It protects market participants who naively expected that deep liquidity would always be there for them, while eliminating any benefits for the market participants who actually were willing to provide that liquidity in the midst of the turmoil. ...

I see merit in both sides of this argument. My economist side thinks people should be responsible for their actions and bear the costs and benefits accordingly. But my, er, human side sees merit in protecting people from trades that seem obviously erroneous.

What’s needed is a compromise–one that maintains good incentives for stock buyers and sellers, but provides protection against truly perverse outcomes.

Happily, the world of insurance has already taught us how to design such compromises: what we need is coinsurance. People have to have some skin in the game, otherwise they become too cavalier about costs and risks. ... Even a little skin in the game gets people to pay attention to what they are doing.

So here is my proposal:  NYSE and Nasdaq should cancel only 90% of each erroneous trade. The other 10% should still stand.

If Jack the Algorithmic Trader sold 100,000 shares of Accenture for $1.00 last Thursday, he should be allowed to cancel 90,000 shares of that order. But the other 10,000 shares should stand–as a reminder to Jack (and his boss) of his error and as a reward to Jill the Better Algorithmic Trader who was willing to buy stocks in the midst of the confusion.

When you lose money in the stock market, even for trades that are obviously based upon and erroneous strategy after the fact, do you get a Mulligan? I must be missing something here, can someone explain why these trades should be canceled?

    Posted by on Monday, May 10, 2010 at 11:43 AM in Economics, Financial System | Permalink  Comments (40)


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