Is tourism really the world's largest industry?

The claim is made in various forms, from industry influencers like Rafat Ali, the founder of Skift, to the UNWTO, which for years has said (in fairness) that tourism is “one of“ the world’s largest economic sectors. We know the claim has been made for at least a decade, thanks to one embattled tourism professor from Northern Arizona University who penned a blog post in 2008 imploring his students to stop parroting the claim.

In order to get the bottom of where this oft-cited statistic comes from—and determine whether it has any merit—you first have to understand how we decide what constitutes an “industry.” It’s an important, albeit unsexy, question.