Materials and Methods
Data
A variety of country and province level figures were extracted from publicly available databases.
The production of meat (metric tons per year) from pigs and poultry, as well as their
respective stocking rates were obtained from FAOSTAT [5]. National Gross Domestic Product
(GDP) estimates, in purchasing power parity (PPP), were obtained by country from the World
Bank database [36] for the reference year 2010, and for the years for which matching poultry
and pig production system figures were available. GDP in PPP was used instead of raw GDP as
it corrects for purchasing power disparities between countries, and is therefore more suitable
for country-to-country comparisons of GDP per capita. For China, sub-national province
GDP estimates (in purchasing power parity) were obtained from the China National Bureau of
Statistics for the year 2010 [37]. The proportions of chickens raised under extensive backyard
production systems (Pext) were data-mined for a number of countries from various literature
sources, with a definition of backyard production systems following that of FAO sector 4, i.e.
village or backyard production with minimal biosecurity and birds and products being consumed
locally [38]. For pigs, a similar assessment was made, but distinguishing extensive (Pext),
semi-intensive (Psint) and intensive (Pint) production systems, according to the definition set
out in Robinson et al. [