Marginal Intra-Industry Trade, a concept originating in international economics, refers to the degree to which the change in a country's exports over a certain period of time are essentially of the same products as its change in imports over the same period.
The concept is therefore closely related to that of intra-industry trade, that being the export and import of the same items, but concerns changes in exports and imports between two points in time as opposed to their values at a given point in time.
The absolute values are needed because these changes in trade flows can sometimes be negative.
Generally adjustment costs or distribution effects are thought to be small if the MIIT index is high.
The choice of the time period to use in making this calculation is somewhat arbitrary but can nevertheless significantly affect the results.