Phase Three Methodology

ISAAC Phase Three, a repeat of Phase One after at least five years, examined variations in time trends of childhood asthma, rhinoconjunctivitis and eczema around the world, and expanded the world maps of these conditions. New centres which did not do Phase One were included in the enlarged worldwide prevalence maps, and a risk factor questionnaire was added, permitting analysis of associations between the three diseases and a range of biomedical, environmental and lifestyle factors.

Phase Three was completed in 237 collaborating centres in 98 countries with a total of 1,187,496 children participating. In the 13-14 year age group 233 centres from 97 countries participated. For the 6-7 year age group there were 144 collaborating centres in 61 countries. The design of Phase Three corresponded to the Phase One study design. The same sampling frame, method of selecting schools and method of selecting children within schools was used.

The Phase Three field work was conducted during 2001-2. The time period between Phase One and Phase Three data collection was designed to be at least five years. This was chosen to be short enough to detect changes in centres where environmental changes may occur rapidly, as in low prevalence countries such as Greece and China, but not too short for centres where environmental changes may occur more slowly, as in high prevalence countries such as New Zealand and USA. 85% of centres conducted Phase Three 6-8 years after Phase One.

The risk factor questionnaire asked questions about diet, height, weight, heating and cooking fuels, exercise, pets, family size and birth order, socioeconomic status, immigration and tobacco smoke exposure. It was an optional component of the study design, so it was not completed in all centres.

Fuller details of Phase Three are published in the Phase Three Manual and in a paper in the International Journal of Tuberculosis and Lung Disease.