These can be a country as a whole, or a region, or a specific population like for e.g. school children, infants or cardiac patients for whom the application of the trial results will be beneficial. There may be initial differences in these populations itself. They may contain urban areas, rural areas, people with different socio-economic status, living in different places with varying geographical and seasonal characteristics and having different life-styles. But these areas or regions may also be inter-related in some aspects.