Adam Hayes
全球市場在過去幾週經理了大幅度的波動，股票價格更是劇烈下跌。由於中國經濟的不確定性、歐元區的貨幣政策和美國可能的加息，投資者出現恐慌的迹象。 美國股票市場的普通股波動由芝加哥期權交易所波動指數（ CBOE Volatility Index）或者波動率指數測量（VIX）。VIX使用標普500指數隐含期權價格判斷對未來30天波動的預期。VIX水平被投資者用來測量投資者信心，有時被稱為“恐懼指數”，價值越高表明不確定性越大。
大蕭條之後，股票市場隨著經濟基本面的恢複緩慢並穩定的上升。事實上，過去六年是有史以來價格波動最低的且是一個持續的牛市。
8月，VIX水平超過40，達到這些年的最高點。雷曼兄弟破產時，VIX上漲到接近80.通常來說，VIX超過20或者30說明投資者正在擔心，低於20則說明投資者對穩定上升的市場有信心。
波動
波動同時測量資產價格運動的多少和有多快，不管方向。換句話說，是對一定時間内價格變化程度的測量。
股票市場波動依賴一個對常態分佈，它能認可股票價格由零區分下降趨勢，但是有無限的上升可能。
人們普遍擔心價格的下降，在同一時間相對於10%的上升更關心10%的下跌。這也是為什麼波動性水平的測量方式，例如VIX，密切與市場下跌趨勢相關。
波動增加預示著近期的經濟衰退、市場下跌或者經濟危機。動蕩的市場可能導致投資者的非理性行為並引發流通性的下降。
因為這些原因，短期投資決定應考慮波動性的增加。但是對於中長期來說，大多數投資者應該忽視市場波動的突然擺動。這裡是原因：
1.波動往往會恢複到平均水平
市場和個人資產的波動，具有均值回歸的特性。實事求是的說，波動將會回歸到歷史的一個平均水平。如果波動處於平均線之上，過段時間之後它會回到那個平均值。如果波動低於平均值，將會有可能上升。
在長期中，波動水平應該趨於平緩並回到平均值。當然，在長時間中，平均水平本身也會波動。這是移動平均水平被當作基準的原因。
可以通過和短期波動水平比較看清這一現象，例如30天或者60天地波動和180-250天或者更長的比較。30天的平均價格可能是27，而180天的變成23.
對於具有長期視野的投資者，波動的加劇不應該引起恐慌交易。
2.波動性可以誘發人類的非理性
從衆心理和成為犧牲品的恐懼可以引起投資者的非理性行為並且在它們低價出售遭受損失的同時增加市場波動。行為金融學解釋感情和認知偏差會導致投資者遭受損失，他們只能等待情況好轉。
人們往往厭惡損失，比厭惡風險更嚴重。個人對損失的反映比相同數量的收益更大。糟糕的是，許多人在面臨損失的時候變成風險尋求者，就像賭徒在輸錢的時候會加大下註。這種非理性行為往往加深實際損失。
巴菲特和 Vanguard的John Boge曾警告投資者坐下來忽略市場波動，以免他們受到受到損失。基於良好的基本面的多樣化投資組合應該能克服短期波動。事實上，全球股票市場現在交易價格比2008年崩盤前價格高。市場自然會波動並返回。
3.熊市使投資者可以購買價格下跌的股票
對於一個擁有良好定義投資策略的投資者，動蕩的市場可以提供好處，讓他們以更低的價格購買股票。例如，每月從收入中購買一定金額股票並不在市場波動中退出的退休儲戶，將會在平均價格上受益，使他們的平均購買價格降低。
能夠保持冷靜並意識到股價在最近的熊市甚至盤中閃電崩盤時股價正在打折的投資者，購買股票以降低他們的平均購買價格。
總結
波動測量價格變化的多少和速度，往往伴隨著投資者近期的恐慌和不確定性。VIX指數，被廣泛用於測量美國整體市場的波動，在過去幾個週VIX水平由於中國、歐洲和本土的不確定性劇烈而上升。
雖然波動可以預測短期熊市和經濟下跌，長期投資者忽視波動性增加並維持他們的策略性投資目標是明智的。波動水平將趨於平均，因此高水平過段時間也將變得平均。投資者恐懼引發的非理性行為將導致損失，甚至使損失更嚴重。一個頭腦冷靜的投資者可以遵循系統化的價格平均方法以更便宜的價格挑選購買和長期持有股票並受益。
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.
本文翻譯由兄弟財經提供
文章來源：
http://www.investopedia.com/articles/investing/091115/3-reasons-ignore-market-volatility.asp