Let me first take issue with two things:

1. I don't think that I or anyone else in this thread have to "prove" that the FT piece was shoddy in order to express that opinion. That said, I do agree that some support is needed.

2. I don't know that FT's data were shoddy, exactly, but Piketty's response certainly suggests that their review was less than thorough and their claims overblown.

That out of the way, Piketty points out a number of things about the FT analysis that, if true, would indeed suggest that the FT analysis was not very good. It is possible that Piketty's statements aren't true -- I'm not an economist, I'm only part of the way through Piketty's book, and I have not looked at the technical papers and datasets exhaustively (though I have looked at them) -- but it all sounds plausible to me, and it is all easily verifiable.

For those who do not want to wade through everything, the basic theme is this: at, it seems, every turn, FT notices an anomaly in Piketty's data and assumes that it reflects an error when, in fact, the anomalies are reflections of conscious methodological choices. FT then makes a big deal out of every "error" even though only one or two of them would make a meaningful difference, even if FT were correct in identifying them. More seriously, in most of these cases, FT could well have answered its own concerns by reviewing the copious technical notes that Piketty has made public. But, it would seem, FT either could not be bothered to consult them, or did consult them but chose not to disclose these explanations to its readers. FT then generates its own results based on its dubious "corrections" of Piketty's "errors" and does not bother to sanity check them. It fails to notice, e.g., that its analysis shows that Britain is more egalitarian than Sweden (hard to believe) and, moreover, that it is one of the most egalitarian societies in the history of the world (even harder to believe). It's not impossible, of course, that Britain actually is that equal, but a responsible researcher, one would expect, would have noticed that feature in its results and made an effort to come to grips with it. FT's failure to approach its own results with that sort of critical eye (the eye with which it read Piketty's results) to me suggests sloppiness or bias.

Some quotes:

...I compute decennial averages on the basis of the closest years available. This is clearly explained in the chapter 10 excel file (see sheet "TS10.1"). For instance, "1870" is computed as the average for years "1873-1877", "1910" as the average "1907-1908", and so on. 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.

The FT also suggests that I made a transcription error by using the estimate for 1908 for the top 1% wealth share (namely, 53.8% of total wealth) for year 1920 (instead of the correct raw estimate for that year, namely 51.5% of total wealth)... In any case, whatever adjustment one chooses to make to deal with this break in series is again going to have a negligible impact on long-run patterns.

The FT argues however that no explanation is given for some of the data construction....This is a surprising statement, because all necessary explanations are actually given in the technical research paper on which these series are based.

What it find somewhat puzzling in this controversy is the following: (i) the FT journalists evidently did not read carefully the technical research papers and excel files that I have put on-line; (ii) 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.

What is troubling about the FT methodological choices is that they use the estimates based upon estate tax statistics for the older decades (until the 1980s), and then they shift to the survey based estimates for the more recent period. 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. Therefore such a methodological choice is bound to bias the results in the direction of declining inequality.

Also note that a 44% wealth share for the top 10% (and a 12.5% wealth share for the top 1%, according to the FT) would mean that Britain is currently one the most egalitarian countries in history in terms of wealth distribution; in particular this would mean that Britain is a lot more equal that Sweden, and in fact a lot more equal than what Sweden as ever been (including in the 1980s). This does not look particularly plausible.