Marketers have a love/hate relationship with ROI. It's absolutely necessary, but annoyingly elusive. A brand's return on investment can be quantified with impressions, engagement or time spent, but rarely direct sales. It's not often that a campaign can be linked causally to a sale, especially purchases at bricks-and-mortar stores. Mobile can change that. What online advertising did for e-commerce, mobile marketing can do for traditional commerce: make it measurable.

This is vastly important right now. In this economic environment, consumers are spending less, switching brands and going online to hunt for deals. Still, almost everyone makes unplanned purchases, and half of those purchase decisions happen in the aisle. Mobile presents a tremendous opportunity for brands to claim the last few feet and turn browsers into buyers.

Advantages of mobile

Today's shoppers are hyper-connected and never leave home without their phones. More importantly, they are using their mobile phones to inform purchase decisions. According to one survey, nearly a third of U.K. shoppers with mobile phones used them while shopping. Sixty-five percent of that group got an opinion about a product and, more often than not, that opinion encouraged them to buy the item.

From an efficiency standpoint, mobile-marketing initiatives need little incremental infrastructure, which makes them inexpensive and easy to scale. They can be made incredibly relevant and drive direct, two-way interaction with the brand. Mobile campaigns that let consumers opt in are also great for CRM and database building. Furthermore, by going direct to the consumer, mobile campaigns allow brands to have more control over their messaging than traditional in-store displays.

Applications for marketers

We have yet to see a brand that is taking full advantage of mobile marketing in-store. Some are taking steps, though, and can be used to illustrate what's possible in the space. Here are our top five applications of mobile in-store:

1. Product information: Customers are using their mobile phones to "phone a friend" and get trusted opinions before shelling out for something. This is the next generation of word of mouth and it is happening in-store, in real time.

   Google Product Search recently launched an application for iPhone and Android phones that allows shoppers to compare product ratings and read reviews through their mobile phones. Sephora, through Bazaarvoice, created a mobile site that allows shoppers to browse product reviews. Consumers are two and a half times more strongly influenced by fellow customer reviews than by a salesperson's advice.

2. Driving traffic: The ability to target by location is one of the most buzzed-about aspects of mobile and one that brands ought to take better advantage of. Where is your nearest store? Where can I buy one of your products right now? By including a mobile prompt on all outdoor creative, marketers can not only increase awareness of their product/service, but they can drive consumers into a store immediately.

   Drilling down even further, location can even be used to target consumers by what aisle they are browsing in a store. Using Wi-Fi hotspots, Acuity Mobile's Aislecaster is accurate up to three to five feet and targets promotions accordingly. For example, a customer in the personal-care aisle might get an incentive to buy face wash.

3. Loyalty programs and mobile coupons: Mobile loyalty programs offer unparalleled targeting and personalization options. Zavers has a system that links online and mobile incentive placement with in-store redemption. The Zavers button can be embedded in any digital media -- internet, mobile, e-mail and interactive TV -- and customers can transfer all these deals directly to a loyalty card on their phone. This means that coupons are now infinitely trackable; marketers can measure the effectiveness of incentives from online ad impression to retail purchase.

   There are also simple SMS-based ways to deliver mobile incentives. Urban Outfitters uses a sign at the register to inform shoppers of their text-to-enter mobile loyalty program. Armani aims to drive more traffic in-store through window signage that gives passersby $10 off if they text in.

4. Campaign extension: To increase brand affinity, stores can offer branded or partner content to consumers. Mobile plus digital out of home (DOOH) is great way to do this. Brands can reach a captive audience as shoppers download content while they are waiting in line. Then they can continue to engage with the brand even after they leave the store.

   Danoo, a DOOH network that places screens near points of sale in coffee shops, is doing just this -- almost all of its in-store campaigns in the fourth quarter of 2008 had a mobile call-to-action. They've seen success for mobile-content downloads through Bluetooth, citing a 30% download rate (out of all people with compatible devices) for a "Dancing With the Stars" campaign.

5. Brand utility: Visa launched a campaign in summer of 2008 to promote the concierge services associated with its new upscale Signature Card. Noting that shoppers are often overwhelmed by wine choices, Visa set up a service where you could text in a food and receive a wine pairing. While this example wasn't necessarily driving sign-up of Visa cards in store, imagine if a winery provided this utility to differentiate itself on the shelf. This idea could apply to many other categories. For example, shoppers could text their complexion to a cosmetics manufacturer and receive recommendations on color choices and makeup application techniques.

Technology will revolutionize retail

Mobile technology is advancing exponentially and can truly help redefine the shopping experience. Here is a sampling of innovations coming soon to a store near you:

• QR codes, or quick-response codes, are becoming standardized in the U.S. and allow direct links to mobile websites via physical codes. Ralph Lauren's youth-oriented Rugby brand has implemented these codes in select stores, letting consumers find more information about a product or order it through their phones.
• By combining the physical and virtual worlds, augmented reality (AR) can give you more information on your surroundings or provide virtual testing. What if you could point your phone at a container and see which of your friends liked it? Applications such as MIT's Sixth Sense, Sekai Camera and GE's Smart Grid are showing the world the potential of the technology -- and it won't be long before you see more brands playing with it.

With these and other emerging technologies, shopping can become a fully connected, social experience. But it's up to marketers, not developers, to define this future. To begin with, we'd like to see more brands take advantage of simple SMS and mobile web technology to enhance and extend their in-store marketing efforts. Those that do will undoubtedly see impressive ROI -- return on innovation.

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Allison Mooney is VP-director of trends and insights at MobileBehavior, Omnicom's mobile strategy and marketing agency. She runs its global trends lab, Next Great Thing.

Christina Lin is a digital-marketing strategist at Tribal DDB, the digital arm of DDB Worldwide. She is a part of the Omnicom M.B.A. Resident program.