Guerrilla customer research is easy. To prove it, we just need 4.5 minutes of your time.

That's how long it takes to watch Harvard Business School professor Clayton Christensen--the guy who coined the term "disruptive innovation"--explain one such technique in a video on the Forum for Growth & Innovation's website.

Christensen (@claychristensen) describes how his team learned why a fast food chain's customers were buying so many milkshakes for breakfast. News flash: It wasn't because they wanted milkshakes.

That's interesting, but it's not the point. The point is how Christensen's team unearthed what motivated this seemingly odd behavior. The method was cheap, simple, and highly replicable. So replicable, we're outlining it in three steps.

1. Before you interview, observe. Christensen's team spent a long day taking notes on milkshake buyers. "It turned out that nearly half of the milkshakes were sold before 8 o'clock in the morning," Christensen says. "The people who bought them were always alone, it was the only thing they bought, and they all got in the car and drove off with it."

2. Ask the big-picture question. The next day, they asked customers "What job are you hiring the milkshake to do?" The answer: The customers "had a long and boring drive to work, and they just needed something to do." They didn't want a milkshake, per se, but something easier to consume than a greasy doughnut or a dry bagel.

3. Conduct research face-to-face. "What job are you hiring the milkshake to do?" is a deceptively complex question. If you ask it in person, you can explain what you mean by "job," ask follow-ups, and drill down. Posed via e-mail, the question might yield a useless answer. Asked in person, it revealed something surprising about commuters' preferences.

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