Financial Times: Say, There Are Quite a Few Problems with Piketty’s Inequality Data

French economist Thomas Piketty’s “Capital in the Twenty-First Century” has been hailed among the left as “groundbreaking” and the “book of the season.” Essentially, the book is about how wealth inequality is going back to levels last seen more than 100 years ago.

But after closely examining the data Piketty used in the bestseller, the Financial Times discovered a series of errors and unexplained data selections that skew his findings.

The data underpinning Professor Piketty’s 577-page tome, which has dominated best-seller lists in recent weeks, contain a series of errors that skew his findings. The FT found mistakes and unexplained entries in his spreadsheets, similar to those which last year undermined the work on public debt and growth of Carmen Reinhart and Kenneth Rogoff.

The central theme of Prof Piketty’s work is that wealth inequalities are heading back up to levels last seen before the first world war. The investigation undercuts this claim, indicating there is little evidence in Prof Piketty’s original sources to bear out the thesis that an increasing share of total wealth is held by the richest few.

Prof Piketty, 43, provides detailed sourcing for his estimates of wealth inequality in Europe and the US over the past 200 years. In his spreadsheets, however, there are transcription errors from the original sources and incorrect formulas. It also appears that some of the data are cherry-picked or constructed without an original source.

For example, once the FT cleaned up and simplified the data, the European numbers do not show any tendency towards rising wealth inequality after 1970. An independent specialist in measuring inequality shared the FT’s concerns.

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When asked by the FT about their findings, Piketty said he used “a very diverse and heterogeneous set of data sources ... [on which] one needs to make a number of adjustments to the raw data sources.

“I have no doubt that my historical data series can be improved and will be improved in the future ... but I would be very surprised if any of the substantive conclusion about the long-run evolution of wealth distributions was much affected by these improvements.”