Walmart's presence actually helps.

• Position: Independent Grocers

It is a widely accepted adage that, when a large, national retail chain enters an urban market, small businesses suffer as a result but, contrary to popular opinion, that is not always the case, according to research from Paul Ellickson, professor of marketing and economics at the University of Rochester (N.Y.), who concludes that independent grocers actually thrive when Walmart enters.

"Our research suggests that Walmart's impact on a particular market structure varies greatly across markets," says Ellickson. "We see that small grocery businesses with a high degree of differentiation in larger markets can actually benefit from Walmart's presence, while the larger grocery chains...