Short answers, pulled from the story.
The Human Development Index was created by Pakistani economist Mahbub ul-Haq. He devised and launched the annual Human Development Reports through the United Nations Development Programme in 1990, with the explicit aim of shifting development economics away from national income accounting toward people-centered measures.
The HDI measures life expectancy at birth, education (combining mean years of schooling for adults and expected years of schooling for children), and gross national income per capita adjusted for purchasing power parity. These three dimensions are combined using a geometric mean to produce a single score between 0 and 1.
The UNDP introduced a new HDI methodology in its 2010 Human Development Report, published on the 4th of November 2010. The new method replaced adult literacy rates and gross enrollment ratios with mean and expected years of schooling, and switched from an arithmetic average to a geometric mean, so a very low score in any dimension pulls the overall HDI down more sharply.
Norway has ranked first on the HDI sixteen times, more than any other nation. Canada has topped the list eight times, Iceland three times, and Switzerland and Japan two times each.
The IHDI is a companion measure introduced in the 2010 Human Development Report that adjusts HDI scores to account for inequality within countries. The 2010 report described the plain HDI as an index of potential human development and the IHDI as the actual level of human development once inequality is factored in.
Economists Hendrik Wolff, Howard Chong, and Maximilian Auffhammer found that data updating errors can misclassify roughly 11% of countries, formula revisions can affect around 21%, and threshold decisions can lead to misclassification of as many as 34% of countries. They recommended the United Nations stop sorting countries into development bins because the cut-off values appear arbitrary.