The study looked at how the underground economy in the U.S. has changed over time. They found that a commonly used measure of this, called the currency ratio, didn't actually show a big increase in underground activity after 1994 when adjusted for certain factors. Instead, changes in the adjusted ratio were mostly due to how interest rates affect currency and deposits. This suggests that claims of a major expansion in the underground economy in recent years might not be accurate and need more evidence to support them.