The geographic area that is the source of the largest and most important proportion of actual users for an event, series of events, organisation or facility. 

Generally speaking, it is preferable for an organisation to decide what is the size of such an ‘important proportion’. But if this is not possible, 80% of the attending audience should be used as a default (see below.)

Where the organisation is a ticketed one, a pragmatic approach here is to base the calculation on postcodes from addresses captured for bookers (unless figures for attenders are available). But if the analysis applies to the catchment for bookers, this needs to be stated. 

For non-ticketed events, postcodes should be collected from a sample of users (e.g. through self-completion surveys, quick and short face-to-face interviews or data generated through a competition). 

When a greater understanding of a facility’s sphere of influence and its impact in attracting attenders is required. Can also be used to substantiate the level of service delivered to the local community.