Personal computer systems ought to participate in an interactive dialog with the user. Reflecting on the real sense of interactivity reveals a disenchanting situation of human-computer interaction. Steward Brand has conducted an interview with Andrew B. Lippman for his book The Media Lab: Inventing the Future at M.I.T. [Brand 87].* * The Media Lab has succeeded the Architecture Machine Group in 1985. It is also led by Nicholas Negroponte.
Lippman defines interactivity as «mutual and simultaneous activity on the part of both participants, usually working toward some goal» [Ibid., p. 46]. Supplementary he nominates five related corollaries.
Interruptibility is required in contrast to a just alternating behavior. Each participant is allowed to interrupt at any time. That does not mean that the active part turns immediately, but the interrupt should at least be acknowledged in a reasonable short period thereafter. Many so-called interactive computer systems miss the point of interruptibility. They offer simply an alternating flow of control.
After interrupting and acknowledging the interrupt it is a matter of granularity when the focus of activity can change to the other participant. For human conversation it is most likely a couple of words or a phrase. A sentence is polite but not common. A paragraph is too long for an interactive conversation. Then the conversation mutates to a lecture. An interactive computer system should aim for smaller grain. Response times have come down from days when computers were operated in batch mode to fractions of seconds for PCs. But slow access to external resources or time consuming calculations make it necessary to think about granularity today.
Graceful degradation is Lippman’s third corollary. It means the ability to handle requests that cannot be answered for the moment. They can and should be suspended without bringing the conversation to halt.
The border between limited look-ahead and non-determination is blurred. A conversation is interactive if it is open to unexpected events. In case of interacting with computers this calls for flexibility. The computer system should not be rigid and force its terms of operation on the user.
Interactivity is often claimed as a property for computer systems. Given Lippman’s understanding of interactivity, those systems should be considered at most as dialogical systems. The active role changes from the user, who is formulating a command, to the computer for executing it, back to the user for the next command, and so forth. An interactive conversation of this kind falls apart, if the response time becomes too long. The user starts to enter a command during a phase when the computer is not ready to accept it. The provision of graceful degradation is applicable in situations where the computer cannot calculate an appropriate result for the given command. Many of the error and alert messages provided by current systems leave the user at a dead end – unsure of how to proceed. They do not offer any explanation or help to achieve the desired result.
Real interactive systems have to be more flexible and less rigid with respect to user interaction.