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    ‘Equal Opportunity’ is a term coined by Harold Thimbleby. It describes something applications often do, but defining it will make us more aware of it, and will encourage us, as designers, to watch for places where it can be usfully employed.
    In many situations the information delivered to us as output can be reused as input. The simplest case on the desktop is the ability to copy text that was output in one window, and paste it as input into another window. This application of the equal opportunity principle saved the user the work of having to read the output, remember it, and then type it in again.

    An example that is more applicable to embedded products is a phone that can display the number of the last caller. This number has been displayed as output. It is very likely that the user will want to call that person back. A design employing the equal opportunity principle will allow the user to apply the number as an input, usually by pressing the ‘Call’ button while the number is visible on the display. The alternative of having to remember the number, and then dial it in would be far more trouble for the user.
    Equal opportunity is not only about redirecting output as input. It also allows controls to be both input and output devices. The volume control on my stereo can be rotated by me to change the volume. A motor within the stereo will rotate it to inform me that the volume has changed, in the case where the volume is changed by the remote control. The dial acts as an input at one time and as an output at another.