An input - output (I-O) model is a matrix of product flows based on a summary classification of all the industries in a region. The idea is that each row and column of the I-O matrix represent an industry group. The cells of the matrix represent the flows of products from one industry group to an other. The diagonal represents the internal flows within each industry group. The structure of the model is aspatial.

However because of industry concentration some spatial information is included. An I-O model could be constructed that had spatial as well as industrial dimensions. Such a model would be very large and would reveal many industrial flows that are not public knowledge. This level of disaggregation is interest for describing transport patterns.

The simplest is the 'regional' I-O model that is based on 'general equilibrium analysis' and captures interindustry feedbacks. It however neglects interregional feedbacks.

A more comprehensive view is included in the 'interregional', or 'multi- regional' I-O models.

The basic data requirements for an interregional model are:
1. estimates of final demands
2. estimates of regional I-O coefficients
3. a set of base-year interregional trade flows
   These will suffice for the point estimate available from the LSG (Lontief-Strout Gravity) model

For many years data requirements and computer capability inhibited the use of interregional I_O models. The data requirements have not relaxed but computer capability is less of a problem.

Both the Transportation Model of Linear Programming and the Gravity model have been used to estimate I_O coefficients and trade flows.

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