Economic anthropology is an empirical science that describes production, exchange and consumption cross-culturally. All societies have economies, but they are variable. Anthropologists evaluate the operations of individual economies and the applicability of Western theories to these cases. Some economic processes work broadly; for example, strategic decision-making, the law of competitive advantage, and calculations of transaction costs help explain many observed patterns. Human economies, however, are often structured as intertwined sectors with distinctive processes. Differences observed in productivity, specialization, institutional structure and social motivations across history and across modern societies are of theoretical significance when constructing the limits of general theory.