Think of a hierarchical pandas index as a set of labeled questions asked in order. Each index level supplies one dimension of row identification: Year, Season, Sport, or Event. section 4.10
Ask a question one level at a time
An Olympic-events DataFrame can use four columns as index levels, then sort rows by Year, Season, Sport, and Event. To find mean athlete age for Summer games from 1936 through 2000, slice the first level by year, select Summer in the second, and leave later levels unrestricted. The same pattern selects Archery teams or Tennis heights. section 4.10
