One of the main reasons mutual funds are the investment vehicle of choice for more than 90 million of us is that they're convenient. Don't have the time or inclination to assemble and follow up on a basket of individual equities? Not to worry: With more than 7,000 mutual funds up and running, you're sure to find plenty that suit your investing temperament and timeline -- too many, in fact.

Fast track
Not just any old fund will do. As I go about the business of separating the wheat from the fund industry's chaff, I zero in on funds that are built to last by using both quantitative and qualitative information. To be sure, that latter set of criteria is far more experiential than numerical, involving assessments of things such as stock-picking strategy, managerial acumen, and the shareholder-friendliness of the shop behind the fund.

But while gauging those aspects takes time and, I'll admit, a certain level of fund geekiness, savvy types can find funds for the long run relatively quickly by focusing on the quantitative side of the ledger. With that in mind, in this and upcoming commentaries, I'll highlight the quantitative "metrics" that matter most to me as the Fool's resident fund geek.

Rearview mirror
Let's start with past performance which, alas, is where all too many folks begin and end their research. Don't get me wrong: Past performance is an important tool in any investor's toolkit, but it has to be used in tandem with at least two other "companion variables" -- managerial tenure and relative returns.

Every investor knows past performance is no guarantee of future results, a point especially worth bearing in mind when it comes to mutual funds. That's because a fund's investment-worthiness is a function of its manager, not a quality inherent to the fund itself. So if a fund's impressive performance history has caught your attention, the next question to ask is whether the manager who established that track record is still on the case.

Consider ...
Case in point: Fidelity Magellan (FUND:FMAGX) -- a large-cap behemoth whose eclectic list of top holdings recently included dividend-paying stalwarts like GE (NYSE:GE) and UnitedHealth Group (NYSE:UNH), go-go picks such as Google (NASDAQ:GOOG) and Genentech (NYSE:DNA), and international concerns like Nokia (NYSE:NOK) and Yahoo! Japan.

When Peter Lynch ran Magellan between 1977 and 1990, the fund was unstoppable, delivering a total return in excess of 2,700%. For the five years that ended this past December, however, Magellan's performance was anemic, lagging S&P trackers such as Vanguard 500 Index (FUND:VFINX) and falling to within spitting distance of its Morningstar peer group's bottom quartile.

Given that track record, it came as no surprise when, back in October, the fund's management deck was shuffled, and Fidelity installed one of its most talented skippers at the helm of its sagging flagship.

The Foolish bottom line
The upshot, then, is this: Without gauging it relative to managerial tenure, past performance doesn't tell you a whole heck of a lot about a fund's forward-looking prospects. Relative returns are similarly significant, and I'll take up that topic in an upcoming commentary.

In the meantime, if you'd like to take a look at how we've combined quantitative and qualitative analysis to assemble a market-walloping collection of outstanding mutual funds, just click here for a risk-free trial of Motley FoolChampion Funds. The newsletter's complete list of recommended funds, world-class discussion boards, model portfolios, and back-issue archives are yours to peruse for 30 days. You're under no obligation to subscribe, so give it whirl and please let us know what you think.

Shannon Zimmerman is the lead analyst for the Fool's Champion Funds newsletter service and doesn't own any of the companies mentioned. UnitedHealth Group is a Motley Fool Stock Advisor recommendation. The Fool has a strictdisclosure policy.