It is based on detailed analysis of extracts from the Inter-Departmental Business Register (IDBR), maintained by the Office for National Statistics (ONS).
Mae’n seiliedig ar ddadansoddiad manwl o ddarnau o’r Gofrestr Ryngadrannol Busnes (IDBR), a gynhelir gan y Swyddfa Ystadegau Gwladol (SYG).