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How are the similar postcode districts identified? Unit postcodes such as "NW6 1XN" can be merged together into postcode districts (e.g. "NW6") and then into still bigger postcode areas (e.g. "NW"). Therefore, a series of unit postcodes can be aggregated into postcode districts. The number of unit postcodes falling into each E-Society Group is calculated for each district. Dividing each Group total by the total number of unit postcodes in each district gives the proportion of the district falling into each Group. Each Group figure may be compared with the corresponding figure (i.e. the proportion of postcodes falling into each Group) for Great Britain as a whole. For each of the eight E-society Groups, a District Index Score is created by comparing the proportion of the unit postcodes falling into the Group with the corresponding proportion for Great Britain as a whole. Thus if a district has four of its 20 unit postcodes falling into (hypothetical) Group N, while nationally 160,000 of the total 1.6 million postcodes fall into that Group, the District Index Score is [(4/20)/(160,000/1,600,000)] = 2. That is to say that Group N as twice as well represented in that district as in Great Britain as a whole. The District Index Scores thus created show the over- or under-representation of each of the eight E-Society Groups within each postcode district. Each postcode district has 8 scores, one for each E-Society Group. The 8 scores for each postcode district are compared with the 8 scores for all other postcode districts, and absolute differences between them are calculated. These difference are them summed across all 8 groups. The lower these scores, the more similar the area, and it is these scores which are ranked to identify the top ten most similar postcode districts. How are the maps created? The maps are created from the District Index Scores for each of the 8 E-society Groups. These scores are divided into five natural Jenks categories along the scale from "least" to "most". The scores show the representation of each Group at the postcode district scale. |