The data above were extracted from Thomson Scientific’s Essential Science Indicators database. This database surveys only journal articles (original research reports and review articles) indexed by Thomson Scientific. Articles are assigned to a category based on the journals in which they were published and Thomson Scientific’s journal-to-category field-definition scheme. Both the articles tabulated and the citation counts to those articles are for the period indicated. Scientists publishing large numbers of papers have a greater likelihood of collecting more citations than scientists publishing fewer papers. This ranking is by total citations. For articles with multiple authors, each author receives full, not fractional, citation credit. Another ranking could be based on citations per paper, which reveals weighted impact. Essential Science Indicators lists authors ranked in the top 1 per cent for a field over a given period, based on total citations. For these data, 2,1 authors are listed in the field of agricultural sciences, meaning that a total of approximately 212,700 authors’ records were reviewed to obtain these results. For more information on Thomson Scientific’s Essential Science Indicators, see http://scientific.thomson.com/products/esi. Also see: http://scientific.thomson.com.