Abstract
Since the advent of graphical user interfaces, electronic information has grown exponentially,
whereas the size of screen displays has stayed almost the same. Multiscale interfaces were
designed to address this mismatch, allowing users to adjust the scale at which they interact
with information objects. Although the technology has progressed quickly, the theory has
lagged behind. Multiscale interfaces pose a stimulating theoretical challenge, reformulating
the classic target-acquisition problem from the physical world into an infinitely rescalable
electronic world. We address this challenge by extending Fitts’ original pointing paradigm:
we introduce the scale variable, thus defining a multiscale pointing paradigm. This article
reports on our theoretical and empirical results. We show that target-acquisition performance
in a zooming interface must obey Fitts’ law, and more specifically, that target-acquisition time
must be proportional to the index of difficulty. Moreover, we complement Fitts’ law by
accounting for the effect of view size on pointing performance, showing that performance
bandwidth is proportional to view size, up to a ceiling effect. The first empirical study shows
that Fitts’ law does apply to a zoomable interface for indices of difficulty up to and beyond 30
bits, whereas classical Fitts’ law studies have been confined in the 2-10 bit range. The second
study demonstrates a strong interaction between view size and task difficulty for multiscale
pointing, and shows a surprisingly low ceiling. We conclude with implications of these
findings for the design of multiscale user interfaces.