Economists Hendrik Wolff, Howard Chong and Maximilian Auffhammer discuss the HDI from the perspective of data error in the underlying health, education and income statistics used to construct the HDI. They identified three sources of data error which are due to (i) data updating, (ii) formula revisions and (iii) thresholds to classify a country’s development status and conclude that 11%, 21% and 34% of all countries can be interpreted as currently misclassified in the development bins due to the three sources of data error, respectively. The authors suggest that the United Nations should discontinue the practice of classifying countries into development bins because - they claim - the cut-off values seem arbitrary, can provide incentives for strategic behavior in reporting official statistics, and have the potential to misguide politicians, investors, charity donors and the public who use the HDI at large.[citation needed] In 2010 the UNDP reacted to the criticism and updated the thresholds to classify nations as low, medium, and high human development countries. In a comment to The Economist in early January 2011, the Human Development Report Office responded to a January 6, 2011 article in the magazine which discusses the Wolff et al. paper. The Human Development Report Office states that they undertook a systematic revision of the methods used for the calculation of the HDI and that the new methodology directly addresses the critique by Wolff et al. in that it generates a system for continuous updating of the human development categories whenever formula or data revisions take place.
 In what year did the ULNDP respond to the criticism of its report?