There was a time not so long ago that we knew definitively which games had sold the most copies. If you wanted to know how many people bought Splinter Cell vs. Kingdom Hearts in the fall of 2002, the numbers were right there (opens in new tab), provided by a market research company called the NPD that got its sales data directly from retail stores. Mathe-magic! 

20 years later we’re still comparing games to each other, but we now live in the age of service games, where the biggest games are free-to-play and sales numbers don’t tell the whole story. We no longer measure a game’s health in millions of dollars, but in how many millions of people are playing it.