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# Long description

This figure has two parts. The upper diagram shows a basic input–output model consisting of three boxes arranged horizontally. The first box labelled ‘input’ is linked by an arrow to the second box labelled as ‘transformation’. This second box is linked by an arrow to the third box, labelled ‘output’.

The lower diagram is the same, but has an additional arrow extending from the output box which returns to the other two boxes as a feedback loop.