近市场表现一直很奇怪，各只股票的走势实际上竟然是一致的。
不过，这种态势不太可能永远持续下去，一些投资者开始构建交易，以便在市场转为正常的时候获益。
“相关性”是投资者们用来形容股票走势一致程度的术语。相关性为100%意味着各只股票都和大盘走势一致。相关性可分为两类：一类是基于历史数据的实际相关性，一类是基于期权交易者对未来相关性预期的“隐含相关性”。
目前，期权市场的隐含相关性在80%以上，意味着标准普尔500指数成份股中有80%的股票将与大盘走势相同。
一般来讲，当股市动荡减弱──就像过去几个月一样，大盘的隐含相关性也会降低。不过，这次不是这样。
过去三个月中，芝加哥期权交易所波动指数(VIX)跌了近一半。然而，据该交易所的数据，股市的隐含相关性几乎丝毫未动，5月份的高点为78%，如今仅仅跌到73%，仍远高于4月份56%的水平。
实际上，现在标准普尔500指数的隐含相关性甚至比2009年初金融危机顶峰时还要高。
目前的一些较高相关性可能是永久性的，这是近年来投资者大规模地从个股转向交易所买卖基金(ETF)造成的。
尽管如此，一些专业人士认为相关性会下降。
野村证券国际(Nomura Securities International)美洲指数交易负责人马尔霍特拉(Rajesh Malhotra)说，我很吃惊相关性保持如此之高的水平，在某个时候，相关性应该朝着50%回归。
专业人士利用高隐含相关性的一个途径是使用所谓的“离差交易”的套利战略：买进一只股票的期权，并卖出一只交易所买卖基金的期权。他们赌的是随着时间的流逝，这二者之间的相关性会减弱。
普通投资者利用离差战略的一个略微容易一些的方法是，使用略微调整的“有抵补看涨期权”战略。通常来讲，有抵补看涨期权是买进一只股票，然后卖出这只股票的看涨期权，这样买家就有在股票触及某个价位时买进股票的权利。
通过卖出看涨期权，他们可以即时获得溢价，帮助防范买进的股票可能遭受的损失。有抵补看涨期权是一种在上涨时获得一定增值、在下跌时提供一定保护的方法。
离差交易
为利用离差交易，你可以稍微调整一下有抵补看涨期权战略：买进一只个股，同时卖出一只交易所买卖基金的看涨期权。为什么这样做？因为在高隐含相关性时期，交易所买卖基金的溢价往往比个股高，交易可能获利更多。
纽约机构经纪自营商Ticonderoga Securities股市衍生品董事总经理马丁(John Martin)说，隐含相关性较高时，你在卖出交易所买卖基金指数期权时能获得额外的风险溢价。
以Market Vectors Gold Miners ETF（简称GDX）的期权为例。该交易所买卖基金跟踪黄金生产商的股票。据Ticonderoga的数据，截至7月27日，GDX期权平均波动幅度为 34%，而它的两只最大成份股Barrick Gold Corp.和Newmont Mining Corp.的期权体现出来的隐含波动幅度分别为33.4%和31.5%。
举例来讲，如果投资者在55.75美元的价位买进Newmont Mining股票，然后以2.40美元的期权价格卖出1月份行使价为62.50美元的看涨期权，锁定的最大收益为16.5%。不过，他们也可以以54美元 的价格卖出GDX，收益17%。在一个聪明交易的世界中，这是一个可以利用的巨大差额。
“波动”基金
有一类被称为“波动”基金的对冲基金常常会进行相关性和离差交易。5、6月份隐含相关性飙升至80%时，一些离差交易员遭受了损失。不过，他们却有望在未来相关性下降时获益。
对冲基金MM Capital驻纽约负责人波顿(Ben Borton)说，鉴于市场的独特状况，现在是做离差交易的好时机。最糟的情况是相关性非常高，而这基本上都已经体现在价格中了，因此从目前水平下行的风险有限。
与此同时，共同基金投资者可以从隐含相关性收获一条简单的经验：当芝加哥期权交易所隐含相关性指数从低到高时，押注指数要好过将希望寄托于活跃的基金经理，因为更多的股票将走势一致。相反，当隐含相关性指数从高到低时，就应该选择将赌注下在活跃的经理身上。
投资公司Jefferies股市衍生品负责人西尔伯(David Silber)说，隐含相关性可以显示人们是在交易个股还是大盘。在某个时候，你会看到人们回到个股。
The market has been behaving strangely recently, with stocks moving in virtual lockstep with one another.
But that isn't likely to last forever, and some investors are setting up trades designed to benefit when the market returns to normal.
'Correlation' is the term investors use to describe the degree to which stocks trade in tandem. A reading of 100% means every stock moved with the index. There are two kinds: actual correlation based on historical data, and 'implied correlation,' based on options traders' expectations of correlation in the future.
Right now, the options market puts implied correlation at more than 80%, meaning that eight out of 10 stocks in the Standard & Poor's 500-stock index will move in the same direction as the index.
Typically, when stock-market volatility falls, as it has in the past few months, so does the overall market's implied correlation. Not this time.
The Chicago Board Options Exchange Volatility Index, or VIX, has fallen by almost half during the past three months. Yet the market's implied correlation has barely budged, falling from a high of 78% in May to 73% now, still well above its level of 56% in April, according to CBOE data.
In fact, the implied correlation for the S&P 500 is higher now than it was even during the peak of the financial crisis in early 2009.
Some of today's higher correlations could be permanent, a result of the huge shift in recent years toward ETFs and away from individual stocks.
Still, some pros are betting that correlations will fall.
'I'm surprised correlation has stayed this high,' says Rajesh Malhotra, head of index trading, Americas, at Nomura Securities International. 'At some point it should move back toward 50%.'
One way the pros try to take advantage of high implied correlation is by using an arbitrage strategy called a dispersion trade: They buy options on an individual stock and sell options on an ETF. The bet is that the two will become less correlated over time.
A slightly easier way for ordinary investors to play the dispersion theme is by using a variation of a 'covered call' strategy. Normally a covered call involves buying a stock and then selling a call option on that stock, giving the buyer the right to buy the stock if it hits a certain price.
By selling the call they get to collect a premium upfront, which helps protect against losses on the stock they bought. A covered call is a way to get some upside while protecting some on the downside.
Betting on Dispersion
To bet on dispersion, you can tweak the covered-call strategy slightly: You buy an individual stock and simultaneously sell a call on an ETF. Why? Because during periods of high implied correlation the premium on the ETF is typically higher than it would be for the individual stock, making the trade potentially more profitable.
'When implied correlation is high, you get extra risk premium to sell the ETF-index option,' says says John Martin, a managing director for equity derivatives at Ticonderoga Securities, a New York institutional broker dealer.
Consider options on the Market Vectors Gold Miners ETF, or GDX, which tracks the stocks of gold producers. As of July 27, GDX options were trading with an average volatility of 34%, while options on two of its largest components, Barrick Gold Corp. and Newmont Mining Corp., priced in implied volatility of 33.4% and 31.5%, according to Ticonderoga.
Investors who buy, say, Newmont Mining at $55.75 could sell January $62.50 calls at $2.40 and lock in a maximum gain of 16.5%. But they could sell the equivalent 54 in GDX instead and pick up 17%. In the world of fast-fingered trading, that is a big gap to exploit.
'Volatility' Funds
There is a category of hedge funds called 'volatility' funds that typically make correlation and dispersion bets. Some dispersion traders took a hit in May and June when implied correlation surged to 80%. But they could be in good position to capitalize on falling correlation in the future.
'It is a good time to be doing dispersion because of the unique conditions in the market,' says says Ben Borton, a principal at hedge fund MM Capital in New York. 'The worst case is that the world will be very correlated, and that is largely what is priced in, so the downside is limited from these levels.'
Mutual-fund investors, meanwhile, can glean a simple rule of thumb from implied correlation: When the CBOE's Implied Correlation Index moves from low to high, indexes are a better bet than active fund managers, because more stocks will be moving in unison. Conversely, when the Implied Correlation Index moves from high to low, active managers are the way to go.
'Implied correlation is an indication of whether people are trading individual stocks or the stock market,' says David Silber, head of equity derivatives at Jefferies. At some point, 'you'll see people move back into single stocks.'