Conclusions

Impact for practitioners
The results of this experiment show the importance of keeping the user informed. A user's command should always be met with an immediate response from the system, otherwise the user might believe the command wasn't received. This practice goes for all commands which a user might invoke including exiting a program. Multiple calls to exit a program might not be viewed as troublesome until excessive "exit" command-line entries or excessive mouse clicks on the underlying desktop are considered.

Theory
The number of excessive command calls appears to be based almost entirely on whether there's any indication that the program is acting on the user's command. All of the treatments that showed feedback had a statistically equivalent probability of reducing the number of excess command calls during the email test. Perhaps part of the reluctance of users to continue issuing commands was due to the separate feedback window which blocked part of the email client's text box. A less intrusive approach -such as a change of icon, or separate feedback zone- might create less disturbance and therefore have less effect on the user.

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