Un bid-ask poco affidabile...

Il Wall Street Journal torna sul tema del trading ad alta frequenza e su come la struttura poco trasparente e troppo fragile dei mercati rischi di allontanare sempre di più i piccoli investitori (oltre al flash crash si sono anche visti ordini che sono capaci di viaggiare nel tempo ...). Scrive Jason Zweig:

For small investors, buying or selling a stock has never been cheaper or easier; the vast majority of the time, small trades cost next to nothing to complete and occur within the blink of an eye.
But every so often, a trade turns into a costly, convoluted nightmare for investors who aren't vigilant to the point of obsessive.
On Thursday, at 2:00:07 p.m., the stock of United Continental Holdings, the airline, was quoted at $19.85 bid, $19.86 ask—meaning that potential buyers were willing to pay $19.85 and potential sellers would part with shares for $19.86.
Within 25 thousandths of a second, however, 142 trades went off in UAL stock, nearly all outside that one-penny "spread," according to Eric Hunsader of Nanex, which analyzes trading data. Someone ended up selling 800 shares of UAL for just $19.54, or 32 cents below the ask price that traditionally marked the maximum point at which a sale would go off.
Such blips, Mr. Hunsader says, happen "dozens of times a day." 
With billions of shares trading hands a day, that makes such events fairly rare. But they are devastating to the confidence of investors. After all, most of us wouldn't want to play Russian roulette even with a pistol that has 999 empty chambers and one chamber with a bullet in it. That is how buying and selling a stock has come to feel for many retail investors.

I problemi sono ancora più seri per quanti utilizzano stop-loss per proteggersi da ribassi improvvisi. Zweig cita il caso di un investitore che ha visto un proprio investimento liquidato automaticamente senza che tuttavia il prezzo abbia mai raggiunto la barriera di protezione. Cosa è successo? Boh!
Nell'attesa che si riesca a dare ai mercati una struttura più stabile Zweig ha alcuni consigli e raccomandazioni che mi sembra opportuno riportare:

So what can you do to trade more safely?
First, avoid open-ended buy and sell orders. The UAL trade was a market order, or an instruction to sell at the best available price. "Rule No. 1 for the small investor is never, ever put in a market order," says Joe Saluzzi, a partner at Themis Trading in Chatham, N.J. Instead, use a limit order that stipulates either the price below which you won't sell or above which you won't buy.
A traditional stop-loss order, as Dr. Penn found, has become dangerous. A "stop-limit" order, combining a stop-loss with a limit below which you won't sell, is a safer approach.