Here is how they determine the rankings. The Greatest of All-Time Top Country Artists, Songs and Albums rankings are based on weekly performance on Hot Country Songs (from its Oct. 20, 1958 inception through June 4, 2016) and Top Country Albums (from its Jan. 11, 1964 inception through June 4, 2016). Titles are ranked based on an inverse point system, with weeks at No. 1 earning the greatest value and weeks at lower spots earning the least. Due to changes in chart methodology over the years, eras are weighted differently to account for chart turnover rates over various periods. Artists are ranked based on a formula blending performance, as outlined above, of all their Hot Country Songs and Top Country Albums chart entries.

http://www.billboard.com/charts/greatest-country-artists