While the size of the market determines the amount of service activity within a city, it may have more service activity than indicated by the size of the market, and this surplus of facilities is called "centrality". A high index value of centrality implies that the city is serving an extensive region outside the city, as well as the urban market itself. Conversely, a deficiency of service facilities suggests that the city's external role is quite limited, or that it may even import goods and service from nearby centres. In general the agricultural centres of western Canada have the highest values of centrality, while the lower values are found in industrial cities of central Canada (Ontario and Quebec) or isolated resource towns. Centrality implies an extensive and well-populated service area.