For instance, as noted in the section on rural and urban water competition, water-trading systems, despite their recognised economic efficiency, are mainly used intrasectorally (OECD, 2010) and rarely driven or used by cities. This discrepancy may originate from multiple factors, mostly related to ease of implementation: the cost of entry and the institutional complexity of the approaches may make them unfit for smaller-scale operations or difficult to establish among users. Moreover, policy makers often decide to favour other objectives - i.e. equity or sustainability - over economic efficiency. Other global, national or local political objectives may also affect implementation: trade liberalisation, food or energy security, land grabbing, poverty alleviation or the strong lobby groups might tilt the scales towards a particular policy choice.