910 ITU‐T's Technical Reports and Specifications Math/science skills attainment Top performers' combined mean scores on the math and science components of an Organisation for Economic Co‐operation and Development (OECD) assessment of 15 year olds' academic preparedness. Top performers are defined as those students who achieved in the top two proficiency levels (Level 5 and Level 6) on the math and science portions of the test. Comparable examinations are used wherever possible to place cities not included in the OECD assessment. Miles of mass transit track Total miles of metro, tram and light rail track within a city divided by the total population and then multiplied by 100,000. Includes monorail and commuter rail that run within a city if they operate as metros in the city. Natural disaster risk Risk of natural disasters occurring in or near a city. Counted hazards include hurricanes, droughts, earthquakes, floods, landslides and volcanic eruptions. Number of global 500 headquarters Number of global 500 headquarters located in each city. Operational risk climate Quantitative assessment of the risks to business profitability in each of the countries. Assessment accounts for present conditions and expectations for the coming two years. The operational risk model considers 10 separate risk criteria: security, political stability, government effectiveness, legal and regulatory environment, macroeconomic risks, foreign trade and payment issues, labour markets, financial risks, tax policy, and standard of local infrastructure. The model uses 66 variables, of which about one‐third are quantitative. Percent of gross domestic expenditure on R&D Total gross domestic expenditure on research and development in 2007 as a percentage of the gross domestic product. Percent of population with higher education Number of people who have completed at least a university‐level education divided by the total population. A university‐level education is set equivalent to a Bachelor's degree or higher from a US undergraduate institution. Political environment Measure of a nation's relationship with foreign countries, internal stability, law enforcement, limitations on personal freedom and media censorship. Purchasing power Measure of the comparative relationship between prices and earnings calculated by dividing net hourly income by the cost of a basket of 122 goods and services, including rent. Quality of living Score based on more than 30 factors across five categories: socio‐political stability, health care, culture and natural environment, education and infrastructure. Each city receives a rating of either acceptable, tolerable, uncomfortable, undesirable or intolerable for each variable. For qualitative indicators, ratings are awarded based on the Economic Intelligence Unit analysts' and in city contributors' judgments. For quantitative indicators, ratings are calculated based on cities' relative performances on a number of external data points. Recycled waste Percentage of municipal solid waste diverted from the waste stream to be recycled.