US federal debt will reach a record 116% of gross domestic product by 2034, up from 93% today. That’s according to the Congressional Budget Office. As the bipartisan Washington agency tasked with being the umpire in disputes over how legislation will affect the economy, its estimates have some credibility. Or, rather, they should have come credibility.

The CBO’s economic forecasts are among the most accurate and reliable you can get. But when it comes to the federal budget, the CBO is more interested in encouraging deficit reduction than delivering accurate insight. How else to explain why over the past 50 years it has routinely overestimated the amount of interest the US government would have to pay to service its debt?