"Our research got a huge push when a television station in the Netherlands asked us to organize a nationwide study on vocabulary knowledge," says lead researcher Professor Marc Brysbaert of Ghent University in Belgium. "The test we developed was featured on TV and, in the first weekend, over 300 thousand Dutch speakers had done it."
The study was done by volunteers online and quickly became so popular that the researchers developed versions in English and Spanish. It gave the team an unprecedented amount of data to work with.
Users were asked whether or not the word shown to them on the screen was real or not. 100 words were shown, with 70 of them being real and another 30 being random word-like assemblages. The words were selected from a list of 62,000 words compiled by the researchers. That means just because the OED has included YOLO and clickbait doesn't necessarily mean they're included here. So, you can say you know 42,002 if you want.