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Dollar-cost averaging: How often should one use it? What criteria to use when choosing stocks to apply it to?
How often should one use dollar-cost averaging? Trivially, a dollar cost averaging (DCA) strategy must be  used at least twice! More seriously, DCA is a discipline that people (typically investors with relatively small amounts of money to invest each month or each quarter)  use to avoid succumbing to the temptation to "time the market".  As mhoran_psprep points out, it is well-suited to 401k plans and the like (e.g. 403b plans for educational and non-profit institutions, 457 plans for State employees, etc), and indeed is actually  the default option in such plans, since a fixed amount of money gets invested each week, or every  two weeks, or every month depending on the payroll schedule. Many plans offer just a few mutual funds in which to invest, though far too many people, having little knowledge or understanding of  investments, simply opt for the money-market fund or guaranteed annuity fund in their 4xx plans.  In any case, all your money goes to work immediately since all mutual funds let you invest in thousandths of a share. Some 401k/403b/457 plans allow investments in stocks through a brokerage, but I think that using DCA to  buy individual stocks in a retirement plan is not a good  idea at all. The reasons for this are that not only must shares must be bought in whole numbers (integers) but it is generally cheaper to buy stocks in round lots  of 100 (or multiples of 100) shares rather than in odd lots of, say, 37 shares. So buying stocks weekly, or biweekly or monthly in a 401k plan means paying more or having the money sit idle until enough is accumulated to buy 100 shares of a stock at which point the brokerage executes the order to buy the stock; and this is really not DCA at all.  Worse yet, if you let the money accumulate but you are the one calling the shots "Buy 100 shares of APPL today" instead of letting the brokerage execute the order when there is enough money, you are likely to  be timing the market instead of doing DCA. So, are brokerages useless in retirement fund accounts?  No, they can be useful but they are not suitable for DCA strategies involving buying stocks. Stick to mutual funds for DCA. Do people use it across the board on all stock investments? As indicated above, using DCA to buy individual stocks is not the best idea, regardless of whether it is done inside a retirement plan or outside. DCA outside a retirement plan works best if you not trust yourself to stick with the strategy ("Ooops, I forgot to mail the check yesterday; oh, well, I will do it next week") but rather, arrange for your mutual fund company to take the money out of your checking account each week/month/quarter etc, and invest it in whatever fund(s) you have chosen. Most companies have such programs under names such as Automatic Investment Program (AIP) etc. Why not have your bank send the money to the mutual fund company instead? Well, that works too, but my bank charges me for sending the money whereas my mutual fund company does AIP for free. But YMMV. Dollar-cost averaging generally means investing  a fixed amount of money on a periodic basis. An alternative strategy, if one has decided that owning 1200 shares of FlyByKnight Co is a good investment to have, is to buy round lots of 100 shares of FBKCO each month. The amount of money invested each month varies, but at the end of the year, the average cost of the 1200 shares is the average of the prices on the 12 days on which the investments were made. Of course, by the end of the year, you might not think FBKCO is worth holding any more. This technique worked best in the  "good old days" when blue-chip stocks  paid what was for all practical purposes a guaranteed dividend each year, and people bought these stocks with the intention of passing them on to their widows and children.