You might think that when it comes to mutual funds, numbers are what matter most. It's true that mutual fund fees are critical to understand. Ultimately, the return you get is what's most important. And when fund managers study companies, they scrutinize lots of numbers.

But words matter a lot, too. At, Roy Weitz cited a striking example of how words can affect a mutual fund's performance. He pointed to the socially responsible Pax World (FUND:PXWGX) and Domini Social Equity (FUND:DSEFX) funds and then noted how each fund described its investment criteria. Pax World said, for example, that it seeks companies that "are not engaged in manufacturing defense or weapons-related products or companies that derive revenue from the manufacture of liquor, tobacco, and/or gambling products." Meanwhile, the Domini fund aims to "avoid securities and obligations of corporations that manufacture tobacco products or alcoholic beverages."

Weitz then noted that when Starbucks (NASDAQ:SBUX) recently signed a deal with Jim Beam, a liquor-making unit of Fortune Brands (NYSE:FO), it affected socially responsible funds in different ways, depending on their wording. Since Starbucks will now "derive revenue" from liquor, it's essentially disqualified and should be sold out of the Pax portfolio. But since Starbucks won't be making the liquor itself, it still qualifies as an investment candidate for the Domini fund.

Socially responsible funds tend to have many more restrictions than other funds do. But all funds, in their prospectuses, have to explain how they invest. This is probably why many of them use fairly vague language, to give themselves a lot of room. Consider the popular Legg Mason Value Trust (FUND:LMVTX) fund, for example, run by the much-admired Bill Miller. In its prospectus, it says:

"The fund invests primarily in equity securities that, in the adviser's opinion, offer the potential for capital growth. The adviser follows a value discipline in selecting securities, and therefore seeks to purchase securities at large discounts to the adviser's assessment of their intrinsic value.. The adviser takes a long-term approach to investing, generally characterized by long holding periods and low portfolio turnover. The fund generally invests in companies with market capitalizations greater than $5 billion, but may invest in companies of any size."

The gist is that the fund managers are looking for undervalued securities -- and value can be estimated in many different ways. They focus on long-term results and expect to hold securities for a long time, an approach that should be appealing to many Fools. The managers generally focus on companies valued at more than $5 billion, but since the word "generally" is in there, a company of almost any size could qualify. This kind of vague language may make an investor worry that there's no focus at the fund, but as long as you trust the managers and have faith in them, the fund description isn't too important.

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Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article.