Expedia CMO David Doctorow says that simply tracking the last time your company touches a customer before a sale is a pretty poor way to understand your mobile marketing efforts.

“The last touch is dead, but we are all plagued by the last touch zombie,” Doctorow said at the MobileBeat conference in San Francisco today. He added that understanding the last touch remains important, but it’s far from the whole picture. And brands are realizing that now.

When you understand how consumers, including Expedia customers, shop for flight and hotel reservations, the point is made clear.

“When customers shop for a flight, they do 48 searches,” Doctorow said.

Those 48 searches can take place at many different times and on many different devices. A customer might perform a search at Starbucks on a smartphone in the morning, but finalize the reservation on an iPad in bed that night. (In fact, Expedia has separate phone and tablet apps to support those very different shopping experiences).

It’s not enough to track the stats around the search just before the purchase. “If you only value that one mobile web click on that device, boy, you’re going to miss so much,” Doctorow said — because that purchase could have been influenced by an ad or a coupon much earlier in the search process.

In order to track which customer touches have a positive influence on the sale, Expedia does lots of testing.

“We really do follow the money — in a direct response environment we will turn things on and shut things off based on whether they’re working,” Doctorow said. “And what’s true today might be a lie tomorrow. So we keep testing — testing is in our DNA.”

“You want to understand the continuity over time, so you want to see your last touch numbers, but it’s not the whole picture,” Doctorow said.