Adriene Hill: The creation of the world’s largest airline: the merger between United and Continental airlines. Late tonight, the two reservations systems will be integrated. In time, they promise, for East Coast travelers to check-in for Saturday flights. But marrying all that data — names, flights, credit card info, frequent flyer miles — is complicated, and expensive.

Marketplace’s Sally Herships reports.

Sally Herships: The reservation systems for big airlines, like Continental and United, are complex. Robert Mann, an analyst for the industry, uses the term man years, instead of man hours, to describe how much work it takes to merge systems like these.

Robert Mann: These tasks have been going on for upwards of 18 months in some cases.

Continental says it’s been training United employees, 15,000 of them, since last fall in how to use its system. Mann says the costs for these switchovers can run into the millions.

Mann: It’s a big deal because it not only includes the reservation that you’ve made, but it includes any changes to the reservation you’ve made. Including…

Seat assignments or special meal requests. And airlines use passengers lists to estimate what’s called a weight and balance calculation. Mann says planes aren’t allowed to take off without itystem. But he says customers flying United this weekend shouldn’t have any reservations about their reservations. Though just in case, he says be sure to print out your boarding pass at home.

In New York, I’m Sally Herships for Marketplace.