More affluent or "upscale" US consumers are driving higher shopper numbers at club stores like Costco and Sam's Club, a study has shown.

Data from market researchers Mediamark Research & Intelligence (MRI) said club stores saw customer traffic rise by 3.6% between September 2006 and April this year.

However, in a sign that even more affluent consumers are tightening their purse strings, club store shoppers with household incomes of at least $75,000 grew by 12% over the same period, the MRI data showed.

"Upscale consumers have always been a large part of club stores' customer base. It's now clear that a greater number of affluent consumers are being careful with their hard-earned dollars during this economic downturn," said Anne Marie Kelly, vice president of marketing and strategic planning at MRI. "Marketers may wish to take note that price-sensitive strategies and messages will increasingly resonate with even the well-heeled."

According to research from MRI, some 39% of US adults said they had shopped in club stores in the last six months. Those shoppers are 26% more likely to have graduated from college and are 32% more likely to hold management, business or financial operations occupations, the researchers said.

Unsurprisingly, parents, battling with an ever-more stretched budget, are 9% more likely than the population as a whole to have shopped in a club store in the last six months, MRI added.