Record companies spend massive promotional dollars on proven megabuck artists like Michael Jackson and Guns N' Roses. And yet, for one week in January, a scruffy up-and-coming band named Nirvana did something that was hard to believe: They outgunned those Big Guys, not to mention Hammer and U2, beating all of them out of the No. 1 slot on Billboard's Top 200 album chart. How could that happen? Well, due to a change in the way it's compiled, Billboard's chart now recognizes only sales, not hype. The big switch came last May, when the trade publication finalized a deal with a New York-based company called SoundScan that supplies its clients with precise, computer- tallied information about what records people buy-and how many of each. Previously, Billboard compiled its charts from verbal reports supplied by young, easily influenced record-store clerks, and if a record company wanted to promote an artist heavily, a little under-the-counter favor here or there- concert tickets, free albums, whatever-might coerce those underpaid clerks into reporting greater sales than actually took place. Which brings us to today's bottom line, the reason there now can be no going back to the old method: Anyone who still doesn't like the new system could very likely have something to hide. Nirvana's success may well be one result of the change. Another could be, as industry sources have been known to suggest, that some of the marketing people who spoke to record-store clerks might be losing their jobs. And yet another is that the record industry now promotes albums by trying to create a huge initial splash. Get the artist's picture and name in the press as much as possible before a new album's release. Whet consumers' appetites, orchestrate a campaign to get them into record stores nationwide the week the album comes out, and hope sales drive the record to a very high chart position, maybe even No. 1-which can then become the basis for still more hype. Capitol Records launched Hammer's current Too Legit to Quit with the biggest marketing campaign in the label's history (a cool half-mil on TV ads alone), and chartered an MGM Grand Air jet (estimated cost: $350,000) to send Richard Marx to five cities in one day to promote the new Rush Street. (But here comes the downside. For all that effort, Hammer has yet to reach No. 1, and Marx, poor Marx-whose previous album topped the chart-never got higher than No. 39.) Another change with the new system: It isn't just who has the No. 1 album- it's who has it by how much. All No. 1 records aren't equal. While Billboard doesn't publish SoundScan's precise sales figures, other media (with the appropriate connections) can and do. Hard-rockers Skid Row were undoubtedly thrilled when their Slave to the Grind debuted at No. 1 last June, but overall record sales were weak then, and Skid Row had only sold 133,000 copies. In mid-December, when record sales were hot, that result wouldn't even have made the top 10. New albums by Metallica and Guns N' Roses also debuted at No. 1 later in the year, selling more than 600,000 and 700,000 respectively; should their success be considered the equal of Skid Row's? And a No. 1 album is no guarantee of long-term success, anyway. While Slave to the Grind is still charting in the lower 60s, Motown act Boyz II Men, whose album came out even earlier, has now sold twice as many copies as Skid Row and remains firmly ensconced in the top 10. They've never hit No. 1, but hey, who would you rather be? Where Billboard's new chart is really making history, though, is in its accurate reporting of things we never knew before. That acts like N.W.A and Ice Cube-on small labels that can't kick in a half-million promotional dollars when the spirit moves them-have a huge and very loyal following who'll drop a dime on them in an instant. That missing-and presumed sleeping-artists like Neil Diamond and Barry Manilow still have a considerable record-buying audience. And most prominently, that a country star like Garth Brooks can have three albums charting at the same time in the Top 40, and stand unexpectedly revealed as the dominant pop success of 1991-thus confounding the mainstream pop world and especially pop radio, which still has no plans to play him until, maybe, the day he replaces John Lennon in the reunited Beatles.