Econometric modelling, so called because of the basic discipline of early protagonists, aims to develop and calibrate mathematical models which explain a particular aspect of consumer behaviour. Often the analysis aims to extend the explanation to different categories of purchaser and vehicle. Not only was it felt that extrapolatory procedures contained too many imponderables and unrealistic assumptions, but the category analysis technique provided the degree of disaggregation necessary to make the results applicable at the local and household level where they were of most benefit in regional transport planning studies. The converse effect of car ownership levels on public transport, which lies beyond the scope of this book, has been the subject of much recent research by TRRL staff and others. As work has continued, an increasingly apparent problem has been the spatial disaggregation of models and projections to produce practical guidelines at the local level.