Abstract：

Sets out twenty-five basic choices that have come to characterize the research on human health, comfort and productivity in indoor environment at the International Centre for Indoor Environment and Energy, Technical University of Denmark, and the rationale behind these choices. The twenty-five basic choices including seventeen straight either/or choices, seven as choices in which neither or both would be a third valid choice, and one(choice four)expressed as six valid choices, defines almost 2 billion different ways(1719926784 ways)of conducting indoor environmental research. Making a choice of method that differs from the one indicated by the rational described in this paper would radically alter the nature of an experiment and would thus affect its chances of serving a particular purpose. It is essential to identify where different methodological choices have been made when comparing findings obtained at different research centres.