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WHO/Europe established the Childhood Obesity Surveillance Initiative (COSI) in seventeen countries in the Region. The system aims to measure routinely trends in overweight and obesity in primary school children (6-9years), in order to understand the progress of the epidemic in this population group and to permit intercountry comparisons within the European Region.

In 2005 Member States recognized the need for standardized and harmonized surveillance systems on which to base policy development on obesity within the European Region. COSI was a response to this need.

Although each country is free to develop a system that fits its local circumstances, data must be collected according to a common agreed protocol containing core items. The system is designed to be simple, not requiring a major investment of public resources. There is no intention to replace countries’ existing or planned health, anthropometric and dietary surveillance systems. On the contrary, the system should if possible be integrated with them.

The Initiative targets primary school children aged 6.0–9.9 years. Once a nationally representative sample of primary schools is selected, the same schools remain the nationwide sentinel sites for the repeated measurements. Countries may also choose to select a new nationally representative sample at each round. A final effective sample size of 2800 children per age group (6.0–6.9; 7.0–7.9; 8.0–8.9; 9.0–9.9) is determined for each round. Core measurements are body weight and body height; waist and hip circumference are optional, along with associated co-morbidities, dietary intake and physical activity/inactivity patterns. The anthropometric measurements are done by trained examiners and standardized according to the common protocol.

Each country is responsible for its national data collection, funded by local resources, and identifies the institute to be responsible for overall national coordination. WHO develops the protocols and manages the international coordination of the surveillance initiative and facilitates investigators’ meetings.

Each country is asked to sign an agreement with WHO in which it agrees to send a copy of the cleaned data file to WHO/Europe, as well as a detailed report of the data cleaning procedures. Data is analysed at both the country level for the national coordinating centre and the European level (common analyses) by the surveillance initiative investigators team.