去几个月中，股市经历了极度紧张的时期，经历了今年良好的开局后，5月初股市开始下跌，期间“闪电崩盘”加剧了跌势，这种情况一直延续到7月。
随着股价下跌，有关股市调整的传言变为熊市里到处弥漫的言论。有勇气就投资吧，这是肯定的。最近股市随着企业公布一连串强劲的获利报告而活跃起来。
从目前向后回溯（事后诸葛亮不好吗？），今年春天和夏初那些黯淡的日子，尤其是闪电崩盘后的几周，现在看起来是投资的良好时段。虽然不是2009年3月触及的几十年年低点，但却是差不多的低位。
近几个月股市的动荡凸显了长期投资者面临的难题：何时是入市的良好时机？尤其是你有大量资金要投资的时候，比如有了奖金、遗产或出售房屋所得收益的时候。
一 个流行的方法是平均成本法（dollar-cost-averaging），这一方法非常简单：你在一段时间内，每月将一部分资金投入股市（在此情况下， 我们将市场定义为标准普尔500指数的情况），这就是逐渐涉足股市，而不是将所有资金一次都投入股市。随着股市波动，当股价下跌时，按照平均成本法投资将 买入更多股票，而当股价上涨时，则买入的更少。
这跟从工资里扣除一部分钱、或将季度股息进行再投资的每月投资额有些不同，你是在定期向股市投入新钱，这对长期投资者来说是良好的自我要求，只要投资计划是多元化的，包括的不仅是股票就好。
让我们看看大量资金所面临的进退两难的局面吧。一次投入所有资金听起来非常像是抓住了市场时机，而我们知道抓住市场时机几乎是不可能的。鉴于投资的人有数千万，一些局外人将在很长一段时间声称他们有市场时机的判断能力，但那些人也常常翻船，就在你碰巧注意到的时候。
平均成本法会让情况得好些吗？让我们从年初看看。如果你得到了8,000美元圣诞奖金，你是在一年的第一个交易日全都投到股市呢，还是在一年的前八个月每月投资1,000美元呢？
在8月5日股市收盘后，在1月将全部奖金一次性投入股市的投资者涨幅将约为2.2%，在8,000美元的初期股本上获利173.60美元，而用平均成本法进行的投资表现稍好一些，涨幅为2.3%，获利187.13美元。
原因在哪儿呢？投入全部资金的投资者在1月大举买入时，股价相对较高，依平均成本法买入的投资者也一样，但股市在7月跌向低点时，平均成本法的投资者还在买入，因此也以较低的水平买入，这是依平均成本法操作获利稍多的原因。
平均成本法招来许多批评，举例来说，如果股市上涨，立即投资是否明智？这些批评基本上说平均成本法披着华丽外衣，而实际却是判断市场时机，这些批评有一些道理。如果你在去年9月开始了上述买入股票的例子，平均成本法表现将落后，从2009年9月1日起是股市的低迷期。
其他人认为经纪商使用平均成本法以避开客户可能发出的报怨：它不让投资者在接近高点时买入，但也不会让投资者抄底。随着时间推移将投资资金分摊，将减少两者出现的几率。
但平均成本法确实有意义，尤其是在动荡的市场里，这似乎已越来越成为常态。随着时间推移投资一些资金，可以确保你在股市下跌时买入更多股票，在股市上涨时买入更少。
许多投资者犯下了高买低卖的错误，高买是在股市大涨的时候买入，低卖是在人们都惊恐万状的时候卖出。平均成本法可以防范这些毁灭性的非理性行为。单是这一因素就是将其纳入你投资方式的一个原因。
For the past several months, the stock market has gone on one of its white-knuckle jags. After a reasonably good start to the year, prices started skidding in early May with the so-called Flash Crash punctuating a decline that reached into July.
As prices went lower, talk of a correction morphed into bear-market chatter. A nervy time to invest, that's for sure. More recently, of course, stocks have perked up amid a sea of strong corporate profits.
Looking back from today (isn't hindsight wonderful?), those dark days of spring and early summer, especially the weeks after the Flash Crash, now look like pretty good times to have invested. Not as stark as the generational lows reached in March 2009, but lows all the same.
The volatile nature of recent months underscores the long-term investors' conundrum: When is the right time to get into the market, especially if you have a lump sum to invest, such as a bonus, inheritance or proceeds from a home sale?
One popular method involves dollar-cost averaging. The notion is elegantly simple: Rather than taking the plunge with all of your money at once, you invest a set amount in the market (in this case, we define the market as the Standard & Poor's 500-stock index) each month over a set period of time, thereby sort of wading into the market. As prices fluctuate, the dollar-cost averager will buy more of the market when prices are low, and less when prices are high.
This is modestly different from investing a set amount each month as part of a payroll deduction plan or a quarterly dividend reinvestment plan. You are putting fresh money into the market on a regular basis, something that is a good discipline for a long-term investor-as long as the investment plan is diversified to include more than just stocks.
But let's get back to the lump-sum dilemma. Putting it in all at once sounds suspiciously like market timing. And we know that timing the market is practically impossible. Given the tens of millions of people who invest, some outliers will get lucky for long enough to claim market-timing prowess, but those people often stumble just as they get noticed.
Does dollar-cost-averaging make things better? Let's look at things from the start of the year. If you had gotten an $8,000 Christmas bonus, would you have been better off investing it all at once on the first trading day of the year, or investing $1,000 a month over the first eight months?
After the market closed on Aug. 5, the investor who took the January plunge would have been up about 2.2%, or $173.60 on the initial $8,000. The dollar-cost averager would have fared fractionally better, with a 2.3% gain, or $187.13.
The reason? The plunger bought in January when the market was relatively expensive. The dollar-cost averager did, too, but also scooped up the market as it plunged toward lows in July, thereby buying at cheaper levels as well. That's where the dollar-cost averager gained an edge.
Dollar-cost averaging has plenty of critics. For instance, if the market goes up over time, doesn't it make sense to get the money invested right away? This criticism, which basically says dollar-cost averaging is market timing in fancy dress, has some merit. If you started the above example back in September of last year, the dollar-cost averager would come out behind, since Sept. 1, 2009, represented a low point for the stock market during the period.
Others argue that brokers use dollar-cost averaging to ward off the potential wrath of their clients: It prevents the investors from buying at a near-term top -- but it could also rob them of buying at a bottom. Spreading the investment of a lump sum over time diminishes the likelihood of both.
Still, dollar-cost averaging does make sense, especially in volatile markets, which seem increasingly like the norm. Investing a set amount of money over time ensures that you will get more of the market when it is cheap and less of the market when it is expensive.
Many investors make the mistake of buying high-when things are going great-and selling low-when everybody's in a panic. Dollar-cost averaging guards against these damaging animal spirits. That alone is a good reason to have it in your investment quiver.