[1] Together with Jon Beaverstock and Richard G. Smith, they create the GaWC's biennial categorization of world cities into "Alpha", "Beta" and "Gamma" tiers.

The categorization is based upon the author's views of "international connectedness", primarily shown through a regions advanced services firms, such as in accountancy, finance and law).

[3] The GaWC inventory ranks city economics more heavily than political or cultural factors.

The classification results for 2020 are derived from the activities of 175 leading firms providing advanced producer services across 802 cities worldwide.

[10] Alpha level cities are linked to major economic states/regions and highly integrated into the world economy.