Capital in the 21st Century: A Response to FT’s Data Problem

Professor Thomas Piketty has given a more detailed response to the Financial Times articles and blogs on his wealth inequality data in Capital in the 21st Century(here, here, here and here). He says it is “simply wrong” to suggest he made errors in his data.

There are a few things on which we agree. First, the source data on wealth inequality is poor. I have written that it is “sketchy” and Prof Piketty says it is “much less systematic than we have for income inequality”. Second, it would have been preferable for Prof Piketty to have used a more sophisticated averaging technique than a simple average of Britain, France and Sweden to derive an estimate for European wealth inequality. Third, the available data suggests a broad trend of reduction in wealth inequality during most of the 20th Century.

There are more aspects on which there remains disagreement. Prof Piketty does not explain the multiple missing data points in his data or tweaks to it; he explains transcription errors as deliberate adjustments to overcome discontinuities in data, but does not provide formulas or an explanation of why these undocumented adjustments should apply to only one data point in a time series; he does not explain why it is consistent to favour household surveys over estate tax records for the US but not the UK; nor why his UK series showing rising wealth inequality differs so materially from his source materials, which show falling UK wealth inequality in eight of the most recent nine decades.

Further debate on many of these items is difficult because Prof Piketty accepts he makes still undocumented adjustments to his data from his original sources, but says they are appropriate because any alternative would not be plausible. The sources he has appear to be secondary to Prof Piketty’s prior expectations of what the data needs to show.

I would urge those interested, who have the time, to read Prof Piketty’s technical annexes, the source data and the spreadsheets and decide whether his graphs on wealth concentration are a reasonable summary of the sources he used.

This is a fascinating and important debate.

FT