Over the years there has been much research and investigation into the question of change in input—output (I0) coefficients, which lie at the heart of any I0 model. This research has taken many productive directions. Besides technical progress, two main reasons for changes in I0 coefficients have been identified:

    Input factor substitution (including substitution of domestic products by imported commodities) caused by changes in the input price system (price effects), and

    Changing output structures of the industries concerned (product-mix effects).