Retailers must answer the call to make mobile shopping easier and more engaging or they risk getting disconnected from the majority of mobile device users.

While 89.7 percent of the U.S. population aged 18 to 64 have mobile phones, only 49.1 percent are using their phones to shop, according to Arc Worldwide, the marketing services arm of advertising agency Leo Burnett.

Mobile shoppers are using phone-friendly versions of websites and apps to compare prices, read reviews, check out product features, download coupons and make purchases.

Shoppers who used to research big purchases for days can now look up information in a matter of minutes on their phones, while impulsive purchases, such as buying a cup of coffee at Starbucks, can become more considered if people use apps to find a shop, check nutrition data or pay by phone.

It's going to be hard to find a category that is not impacted by mobile shopping, said William Rosen, president and chief creative officer of Arc Worldwide.

The company surveyed 1,800 U.S. mobile shoppers and found that 80 percent are light mobile shoppers, who use their phones less often than the 20 percent of heavy users that many apps are geared toward.

If these light mobile shoppers really start engaging and evolve into heavier mobile shoppers, that's going to increase the mobile shopping population by 50 percent, said Molly Garris, digital strategy manager at Arc Worldwide.

(For a comparison of light and heavy mobile shoppers in Arc Worldwide's survey, click here: http://r.reuters.com/zyx68r).

PATHS TO PURCHASE

Increasing sales of smartphones have implications for retailers ranging from sites such as Amazon.com Inc to more traditional outlets such as Macy's Inc, said Rosen.

The idea of a single path to purchase is dead, he said. There are many paths to purchase, and mobile technology is enabling people to shop in different ways, (and) take different routes to a transaction, than we've ever seen before.

According to Arc Worldwide, retailers and manufacturers should promote their mobile presence in traditional media and in stores to keep shoppers from heading elsewhere.

There is the risk of them using someone else's app and literally getting snatched out of the aisle, Rosen said.

Fifty-one percent of shoppers are more likely to buy from retailers with mobile-specific websites, yet only 4.8 percent of retailers had them, according to a November report from Brand Anywhere and Luth Research.

Some retailers, such as LVMH's Sephora and Target Corp, already do a great job by offering creative apps that heavy users of mobile technology download to their phones, and mobile versions of websites for light users, Garris said.

Plus, as smartphones become more popular, the 40.6 percent of adults with mobile phones who do not yet use them for shopping are a huge group that retailers can target.

Nielsen has projected that more than half of U.S. wireless subscribers will own smartphones by the end of the year, up from 21 percent at the end of 2009.

Phones based on Google Inc's Android software are the leaders in the U.S. market. Apple Inc's iPhone was the single most popular phone used by heavy mobile shoppers surveyed, followed by Research In Motion Ltd's Blackberry, and then phones based on Android software.

Leo Burnett is part of Publicis Groupe SA.

(Reporting by Jessica Wohl; editing by John Wallace)

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