20%: About how much lower a country’s output would be after 23 years of slower growth.

Slow economic growth has large cumulative effects, though not quite as big as claimed by two Harvard economists in their response to a critique this week.

This week Carmen Reinhart and Kenneth Rogoff came under fire over their well-known study of the relationship between economic growth and a nation’s public-debt levels. Part of the critique dealt with calculation errors made in Excel, while also challenging the economists’ choice of data to analyze.

In their response to the criticism, Ms. Reinhart and Mr. Rogoff acknowledge the Excel mistake, but stand by their data selection. The economists also note that the critique finds a relationship between high debt and slower growth. (Though, whether that connection has any causality is up for debate.) The critique finds a one percentage point difference in average growth between high- and medium-debt countries, not as large as Ms. Reinhart and Mr. Rogoff found but still substantial. In their response, Ms. Reinhart and Mr. Rogoff say: “If a country grows at 1% below trend for 23 years, output will be roughly 25% below trend at the end of the period, with massive cumulative effects.”