The date was chosen as the study's starting point because the earliest Forbes 400 data on American billionaires dates back to 1982.
The study examined how billionaires' fortunes would have been affected both by moderate and extreme wealth taxes. For the purposes of this article, Business Insider looked at the moderate wealth tax only, which would tax fortunes over $1 billion at 3%.
As the study's authors noted, "The wealth tax has a much larger cumulative effect on inherited and mature wealth than on new wealth," which helps explain the differences in how extremely some of the billionaires would be affected.
However, the study's model fails to account for the fact that ultra-wealthy Americans will likely reengineer their finances to try to evade the tax. That's a real possibility, former Department of Justice tax attorney James Mann, who is now a tax partner at law firm Greenspoon Marder, told Business Insider in August.
Warren's estimates also don't consider tax evasion, Mann said. The authors write that technological advances, the United States citizenship-based tax system, and a high exemption threshold make tax evasion less of a concern.
All data on billionaires' 2018 net worth is sourced from the Brookings study, which relied on Forbes data.