Mr. Piketty acknowledges that adjusting estate tax data to try to assess wealth trends among the living is tricky, but responds that the explanations are indeed in the technical research paper on which the data is based. He writes: “Differential mortality is a complex issue, and we do not have perfect answers; but we do our best to address this issue in the most transparent way. In particular, we put online on this website the large micro files that we have collected in French inheritance archives, so that everybody can reproduce our computations and use this data for their own research.”

Image Thomas Piketty last month in New York at a panel discussion on income inequality. Credit... Karsten Moran for The New York Times

He writes that “The FT journalists evidently did not read carefully the technical research papers and Excel files that I have put online; whatever adjustment one makes to correct for differential mortality (and I certainly agree that there are uncertainties left regarding this complex and important issue), it should be clear to everyone that this really has a relatively small impact on the long-run trends in wealth inequality. This looks a little bit like criticism for the sake of criticism.”

Picking the wrong years for comparison. The Financial Times questions Mr. Piketty’s use of different, arguably random years to stand in for an entire decade, for example 1935 for 1930. Mr. Piketty replies: “The raw series are usually not available on annual basis, so I compute decennial averages on the basis of the closest years available. This is clearly explained in the chapter 10 Excel file.” He also wrote: “These choices can be discussed and improved, but they are reasonably transparent (they are explicitly mentioned in the Excel table, which apparently The FT did not notice), and as one can check they have negligible impact on long-run evolutions.”

What about Britain? The sharpest attacks by The Financial Times — and those that most strongly undermined Mr. Piketty’s thesis about rising wealth inequality — concerned his use of wealth inequality data for Britain. By using more reliable survey data, as opposed to tax receipt data, the newspaper argues that no clear-cut trend on wealth inequality is evident. If true, that would undermine Mr. Piketty’s broader argument that wealth is on an upward march in the developed world, becoming ever more concentrated in the hands of the very rich.

The difference is huge: Using the survey data, as The Financial Times does, results in a finding that the top 10 percent of British households control 44 percent of total wealth in 2010. Mr. Piketty’s preferred source puts that number at 71 percent.

“This is a very large difference indeed,” Mr. Piketty concedes. The problem, he writes, is that for older periods the only data available are studies of estate tax data, including careful research based on estate tax data from the 1920s to 1980s. Mr. Piketty then updated that old research using estate tax data over the decades since then.

He argues that The Financial Times’s approach requires apples-to-oranges comparisons, using estate tax data for older periods and then switching to surveys for more recent information. “This is problematic because we know that in every country wealth surveys tend to underestimate top wealth shares as compared to estimates based upon administrative fiscal data,” he writes. “Therefore such a methodological choice is bound to bias the results in the direction of declining inequality.”