Strictly speaking, the term "catalysis" should not be used unless it can be shown that the number of product molecules produced per number of active sites is greater than one; this is difficult to do in practice, although it is often assumed to be true if there is no loss in the photoactivity of the catalyst for an extended period of time. Reactions that are not strictly catalytic may be designated "assisted photoreactions". Furthermore, phenomena that involve complex mixtures of compounds (e.g. soil) may be hard to classify unless complete reactions (not just individual reactants or products) can be identified.