CHICAGO -- Every year companies spend millions of dollars on focus groups and marketing surveys to get the big picture of their customer base. But they might do well to investigate the individual trees instead of cross-sectioning the whole forest, according to experts at DePaul University. In a technique dubbed Customer Case Research, gleaned from detectives and investigative reporters, companies can uncover what drives customer purchases. A report published in Marketing Research, the journal of the American Marketing Association, describes case studies where companies benefited from observing behavior and asking probing questions until they constructed a detailed narrative of the chain of people, influences and events that lead to specific purchases. For example, the report cites a company that in 1984 developed and marketed a training system that employed a 12-in. laser-disc player, an IBM PC/XT computer and a touch-screen monitor for interactive video training. Although the company had developed a 1000-organization client base, the system's initial launch produced only six customers. Gerald Berstell, who invented the technique and co-authored the report, decided to "interrogate" those six customers for clues. "I concluded that there were six companies that had devised a successful process for buying [the product], and that I needed to learn everything I could from them," he said. What he discovered shook heretofore unquestioned assumptions. First, the system's powerful slogan, "THE Training Technology of the 21st Century," was scaring the very training managers the company was targeting. And even though the company produced nearly 50 training topics, five of the six customers were using only one. So the company changed its focus from a broad-based library to an individually focused solution. Based on this experience, Berstell offers this lesson for those involved in introducing breakthrough technologies: "When you introduce a very new way of doing things, it is easy to come up with a product and marketing approach based on some assumptions that you never even think to question. These faulty assumptions can be fatal and may never be uncovered." Small high-tech companies can design, conduct and complete Customer Case Research in less than a month, he added. It doesn't require hundreds of surveys or a huge budget -- just a magnifying-glass approach and a nose for news.