Adam Hayes
全球市场在过去几周经理了大幅度的波动，股票价格更是剧烈下跌。由于中国经济的不确定性、欧元区的货币政策和美国可能的加息，投资者出现恐慌的迹象。 美国股票市场的普通股波动由芝加哥期权交易所波动指数（ CBOE Volatility Index）或者波动率指数测量（VIX）。VIX使用标普500指数隐含期权价格判断对未来30天波动的预期。VIX水平被投资者用来测量投资者信心，有时被称为“恐惧指数”，价值越高表明不确定性越大。
大萧条之后，股票市场随着经济基本面的恢复缓慢并稳定的上升。事实上，过去六年是有史以来价格波动最低的且是一个持续的牛市。
8月，VIX水平超过40，达到这些年的最高点。雷曼兄弟破产时，VIX上涨到接近80.通常来说，VIX超过20或者30说明投资者正在担心，低于20则说明投资者对稳定上升的市场有信心。
波动
波动同时测量资产价格运动的多少和有多快，不管方向。换句话说，是对一定时间内价格变化程度的测量。
股票市场波动依赖一个对常态分布，它能认可股票价格由零区分下降趋势，但是有无限的上升可能。
人们普遍担心价格的下降，在同一时间相对于10%的上升更关心10%的下跌。这也是为什么波动性水平的测量方式，例如VIX，密切与市场下跌趋势相关。
波动增加预示着近期的经济衰退、市场下跌或者经济危机。动荡的市场可能导致投资者的非理性行为并引发流通性的下降。
因为这些原因，短期投资决定应考虑波动性的增加。但是对于中长期来说，大多数投资者应该忽视市场波动的突然摆动。这里是原因：
1.波动往往会恢复到平均水平
市场和个人资产的波动，具有均值回归的特性。实事求是的说，波动将会回归到历史的一个平均水平。如果波动处于平均线之上，过段时间之后它会回到那个平均值。如果波动低于平均值，将会有可能上升。
在长期中，波动水平应该趋于平缓并回到平均值。当然，在长时间中，平均水平本身也会波动。这是移动平均水平被当作基准的原因。
可以通过和短期波动水平比较看清这一现象，例如30天或者60天地波动和180-250天或者更长的比较。30天的平均价格可能是27，而180天的变成23.
对于具有长期视野的投资者，波动的加剧不应该引起恐慌交易。
2.波动性可以诱发人类的非理性
从众心理和成为牺牲品的恐惧可以引起投资者的非理性行为并且在它们低价出售遭受损失的同时增加市场波动。行为金融学解释感情和认知偏差会导致投资者遭受损失，他们只能等待情况好转。
人们往往厌恶损失，比厌恶风险更严重。个人对损失的反映比相同数量的收益更大。糟糕的是，许多人在面临损失的时候变成风险寻求者，就像赌徒在输钱的时候会加大下注。这种非理性行为往往加深实际损失。
巴菲特和 Vanguard的John Boge曾警告投资者坐下来忽略市场波动，以免他们受到受到损失。基于良好的基本面的多样化投资组合应该能克服短期波动。事实上，全球股票市场现在交易价格比2008年崩盘前价格高。市场自然会波动并返回。
3.熊市使投资者可以购买价格下跌的股票
对于一个拥有良好定义投资策略的投资者，动荡的市场可以提供好处，让他们以更低的价格购买股票。例如，每月从收入中购买一定金额股票并不在市场波动中退出的退休储户，将会在平均价格上受益，使他们的平均购买价格降低。
能够保持冷静并意识到股价在最近的熊市甚至盘中闪电崩盘时股价正在打折的投资者，购买股票以降低他们的平均购买价格。
总结
波动测量价格变化的多少和速度，往往伴随着投资者近期的恐慌和不确定性。VIX指数，被广泛用于测量美国整体市场的波动，在过去几个周VIX水平由于中国、欧洲和本土的不确定性剧烈而上升。
虽然波动可以预测短期熊市和经济下跌，长期投资者忽视波动性增加并维持他们的策略性投资目标是明智的。波动水平将趋于平均，因此高水平过段时间也将变得平均。投资者恐惧引发的非理性行为将导致损失，甚至使损失更严重。一个头脑冷静的投资者可以遵循系统化的价格平均方法以更便宜的价格挑选购买和长期持有股票并受益。
3 Reasons to Ignore Market Volatility
By Adam Hayes, CFA
World markets have seen a spike in volatility over the past few weeks as stock prices have gyrated wildly. Sparked by uncertainty over the Chinese economy, European monetary policy, and the possibility of a U.S. interest rate hike, investors have begun to show signs of panic. General stock market volatility in the United States is measured by the CBOE Volatility Index, or VIX. The VIX uses implied options prices on the S&P 500 index to gauge expectations for 30-day future volatility.The level of the VIX index is used by investors to measure investor confidence, and is sometimes referred to as the "fear index", with higher values indicating greater uncertainty.
For the years following the Great Recession, stock markets rose slowly and steadily as economic fundamentals began to recover. In fact, the past six years have been largely characterized by historically low price volatility and a persistent bull market.
In August, the VIX reached levels in excess of 40, the highest readings in years. To put things in to perspective, the VIX rose to nearly 80 during the Lehman Brothers collapse. Generally speaking, VIX levels above 20 or 30 indicate that investors are worried, and levels below 20 indicate that investors have confidence in stable, rising markets.
Volatility
Volatility measures both how much and how quickly asset prices move, regardless of direction. In other words, it is a measure of the degree of variation of prices measured over some period of time. If asset prices are distributed in a known fashion, such as over a normal distribution, the volatility is measured by determining how many times the price moved a given number of standard deviations from the mean.
Stock market volatility relies on a log-normal distribution, which acknowledges that stock prices are bounded by zero to the downside, but have unlimited upside potential.
People generally fear a downward move in prices, and are much more concerned with a 10% drop than a 10% increase over a similar period of time. This is why measures of volatility levels, such as the VIX, are positively correlated with down moves in the market.
Increased volatility predicts near-term economic downturns, bear markets, or a crisis. Volatile markets can cause investors to behave irrationally and can lead to a decrease in liquidity.
For those reasons, short-term financial decisions should take increased volatility under consideration. But over the mid- to long-term however, it may be wise for most investors to ignore sudden swings in market volatility. Here is why:
1. Volatility Tends to Revert to the Mean
The level of volatility for markets, as well as for individual assets, tends to exhibit the property of mean reversion. Put plainly, there will be an average (or mean) level of historic volatility established over time that future levels will return to. If volatility spikes to levels well above the mean, then over time it should decline back to that average level. If observed volatility levels are quite a bit lower than the average, over time the volatility should be expected to increase.
Over the long run, volatility levels should smooth out and return to the average. Of course, over long time periods, the average level itself may fluctuate. That's why a moving average level is often used as the benchmark.
One can see this phenomenon by comparing short-term volatility levels, such as 30- or 60-day volatility, to longer term volatility levels, such as 180- 250-day or longer. For example, current 30-day historical price volatility for the S&P 500 ETF, SPY, is a around 27 right now, while 180-day volatility is closer to 23.
For investors with long-term horizons, spikes in volatility shouldn't be cause for panic trades.
2. Volatility Can Induce Human Irrationality
Following a herd mentality and falling victim to fear can cause investors to behave irrationally and increase volatility levels as they sell on lows and take losses. Behavioral finance explains how emotional and cognitive biases can cause poor outcomes for investors, who may be better off simply waiting things out.
People tend to be loss averse, rather than risk averse. Individuals respond with greater emotion to a loss than by a gain of equal size. Worse still, many people will become risk-seeking when facing a loss, like a gambler will double down his bets at the casino just with hopes of breaking even. The result of this irrational behavior often deepens actual losses.
Respected professional investors from Warren Buffet to Vanguard's John Bogel have vocally warned the public to sit back and ignore market volatility, lest they fall victim to their own fallibility. A well-diversified portfolio based on sound fundamentals should be able to weather short-term bouts of volatility. In fact, stock markets across the globe are now trading at higher levels than before the crash of 2008. Markets will naturally fluctuate and may post negative returns from time to time. As another example, the S&P 500 has had a compound average growth rate (CAGR) in excess of 10% total return over the past one hundred years.
3. Bear Markets Allow Investors to Buy the Dips
For an investor with a well-defined investment strategy, who invests and re-balances regularly and systematically, volatile markets can actually provide a benefit by allowing them to buy shares at lower prices than they would be able to otherwise. For example, retirement savers who save a portion of their income each month to contribute to a 401(k) or IRA account and do not deviate in volatile markets will benefit from dollar cost averaging, creating a lower overall average purchase price for the shares that they end up owning.
Investors who were able to keep a cool head, and recognize that stock prices were "on sale" during recent bear markets or even intraday during flash crashes, bought shares to improve their average purchase price.
The Bottom Line
Volatility measures how much, and how fast price swings occur, and is often accompanied by investor fear and uncertainty about the near-term. The VIX index, sometimes called the "fear index" is a widely used to measure broad market volatility in the United States, and VIX levels have spiked in the last few weeks as a result of market gyrations caused by uncertainty in China, Europe and at home.
While volatility can be a predictor of short-term bear markets or economic downturns, long-term investors are wise to ignore increased volatility and maintain their strategic investment goals. Volatility levels tend to revert to the mean, so even high levels are expected to return to normal over time. Investors who fall victim to fear may act irrationally and lock in losses, or even make their losses worse. A cool-headed investor who understands this can benefit from volatile markets by following a systematic dollar-cost-averaging approach and pick up stocks at bargain prices to hold for the long-term.
本文翻译由兄弟财经提供
文章来源：
http://www.investopedia.com/articles/investing/091115/3-reasons-ignore-market-volatility.asp