Don't Sell That Stock!

One of the most challenging moments in investing is deciding when to sell stocks out of your portfolio. But did you know that you're probably selling stocks without even realizing it?

When not buying is selling
At last year's annual meeting, Chris Davis, manager of the Clipper Fund (CFIMX), argued that mutual funds essentially sell some of their holdings in a company when they choose not to buy more, and instead add a different holding to the portfolio.

He used Warren Buffett's Berkshire Hathaway as an example, saying, "Every year that they didn't buy Coca-Cola, they sold Coca-Cola. Coke became a smaller percentage of their assets as new money came in and didn't go into Coke."

In other words, as new money comes in and is allocated to new positions, old positions have a smaller and smaller share of the investment pie. So the influence an old position can have on returns gets smaller and smaller, just as if some of that position had been sold.

If you've ever added new money to your portfolio by buying new holdings, the same thing has happened to you.

The big picture
Although we tend to talk about stocks one by one, how they're combined in your portfolio is an important part of investing. It's not enough to just add stocks to your portfolio whenever you run across compelling ones -- you should also be taking into account other factors, such as your overall diversification and the size of various stakes.

Fail to pay attention to the overall makeup of your portfolio, and you might end up with a large chunk of your portfolio in, say, oil-related companies, or pharmaceutical firms -- and if something happens in the industry (imagine the price of oil plunging, for example, or unwelcome health-care reforms), they could all take a big hit together.

But you don't want your portfolio to get too big, either. If you've added so many companies that each one makes up just 1% or 2% of your portfolio, a big home run from one stock isn't likely to make a huge difference to your bottom line.

That's why, as Chris Davis suggested, adding more money and companies to your portfolio, and therefore shrinking the power of your existing holdings, matters.

Perfect your portfolio
Instead of distributing your dollars between the stock that seems most promising, the 56th most promising stock, and all the stocks in between, you want to concentrate your investments on your very best ideas -- even when you add money.

Advisors often recommend holding between eight and 12 (sometimes as many as 20) companies in your portfolio. And that means making sure that every investment in your portfolio really is one of your very best ideas.

What constitutes a very best idea? For my part, it means stocks with strong prospects and strong growth. For example, here are some companies that popped up when I screened at Motley Fool CAPS for large-cap companies rated four or five stars (out of five), with revenue and earnings growth rates of at least 10%, and returns on equity (ROE) of at least 15%:

Source: Motley Fool CAPS.

The Foolish bottom line
You should always be putting your money into your very best ideas -- and if a new idea supersedes an old one, selling a less-good idea to put that money in the very best idea is the right move. But whatever you do, don't sell your stocks without knowing it.

If you'd rather not hunt for promising companies on your own, let us help you. I invite you to check out our Motley Fool Inside Value newsletter, which seeks out undervalued companies to hold for the long term. A free, no-obligation trial will give you full access to all past issues and all recommended stocks. Just click here to get started -- there's no obligation to subscribe.

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This article was originally published on June 1, 2009. It has been updated.

Longtime Fool contributor Selena Maranjian owns shares of Berkshire Hathaway, Intuitive Surgical, and Coca-Cola. Intuitive Surgical and Sohu.com are Motley Fool Rule Breakers picks. Berkshire Hathaway is a Stock Advisor selection. Berkshire Hathaway and Coca-Cola are Inside Value recommendations. Coca-Cola is an Income Investor recommendation. Garmin is a Global Gains pick. The Fool owns shares of Berkshire Hathaway. The Motley Fool is Fools writing for Fools.