Complexity of user interfaces: Can it be educed by a mode key?

Control panels of computer and other modern instruments are often equipped with so-called mode keys, the pressing of which changes the function of other control elements. Thus user keys have different functions depending on the current mode of the instrument. The question is, however, whether it is more userfriendly to have a panel with parallel arranged keys for each function thus with almost direct possibility to intervene , or to have serial equipment with only a few user keys, where the different functions are only available if the user calls up the relevant machine mode e.g. displayed on a monitor that operates with different switchable screens . In this case there exist only serial access possibilities. Two experiments compared performances with three types of user interfaces with and without mode keys on the basis of selection times and errors. Although mode keys apparently reduce the complexity of the user interface, our results show that they lead to slower and more often incorrect usage. However, the amount of practice was a moderator variable. As a consequence, for occasional users it is worth considering a less complex interface, that is, with mode keys, but for expert users an interface where each function has its own key should be preferred.