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52 Chapter 2 Understanding and conceptualizing interaction

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modeling. Key aspects of his conceptual model were: (i) to create a spreadsheet that
was analogous to a ledger sheet in the way it looked, with columns and rows, which
allowed people to capitalize on their familiarity with how to use this kind of repre-
sentation, (ii) to make the spreadsheet interactive, by allowing the user to input and
change data in any of the cells in the columns or rows, and (iii) to get the computer
to perform a range of different calculations and recalculations in response to user
input. For example, the last column can be programmed to display the sum of all the
cells in the columns preceding it. With the computer doing all the calculations, to-
gether with an easy-to-learn-and-use interface, users were provided with an easy-to-
understand tool. Moreover, it gave them a new way of effortlessly working out any