Targets at the edge of the screen
effectively have inﬁnite width
We used the Least-of method of determining target in
two-dimensions, which MacKenzie and Buxton (1992)
found to be comparable to the W’ Model (actual target
depth along the approach vector).
MacKenzie, I. S., & Buxton, W. (1992). Extending Fitts' law to two-dimensional tasks. Proceedings of the ACM
Conference on Human Factors in Computing Systems - CHI '92, pp. 219-226. New York: ACM.
Are movement times lower while
selecting targets at the edge of the screen
than predicted by Fitts’ law?
Does the magnitude of effect vary based
on target size?
Bounded mouse movements will be
faster than Fitts’ Law would predict.
Bounded mouse movements will be
faster than identical unbounded
Simulate the edge of the screen with a
Participants perform an identical set of
pointing tasks with a bounding box and
Presence of Bounding Box
Size of Target
Observed Movement Time
Addressing Potential Confounds
Screen Resolution Consistent at 1680x1050
Subject Distance from Screen Same chair height and distance from monitor
Type of Mouse Use of identical Dell optical mouse
Fatigue Breaks after 25 trials
Order Effects Randomized trials to eliminate order effects
Device LCD with identical calibration and constrast
Starting Position Always in the center of the screen
Potential Confounds What We Controlled
2 Foot distance from Display
Targets are 1º and 1.2º of Visual Angle
Dell optical mouse
Randomized order of trials
10 second break after 25 trials to reduce fatigue
Bright green targets on black background
Pink bounding box
Trial time = Time from start until successful click
0.5s ﬁxation time as cursor is auto-centered.
Cursor always starts at center of screen
8 varying target distances
Two distinct target sizes
Same set of targets
Average(ObservedMT) Average Observed MT vs. Condition
signiﬁcant difference between bounded MT and unbounded MT. almost 100 ms difference.
bounding versus no bounding is not signiﬁcant for large targets,
but, for small targets, the effect is signiﬁcant, and is close to 100ms
No Bounding Box Bounding Box
Correlation between Observed MT and Predicted MT
so, does Fitts law still work? We were trying to break it. It works very well when there is no
bounding box (around .93), and it still works fairly well when there is a bounding box
Observed MT vs. Predicted MT (Large targets with Bounding Box)
This is a line representing what Fitts law predicts, and box plots for all of the observed MTs
at each index of difficulty.
pretty good ﬁt for large targets with bounding box
Observed MT vs. Predicted MT (Large Targets with No Bounding Box)
also a good ﬁt for large targets with no bounding box
Observed MT vs. Predicted MT (Small targets with Bounding Box)
interesting: these boxes tend to be a bit lower than the Fitts law trend line
Observed MT vs. Predicted MT (Small Targets with No Bounding Box)
and here, Fitts law works pretty well again- the bounding box is gone, so it’s just the normal
Differences of Observed Time and Predicted Time
So, there is no signiﬁcant difference between bounding box and no bounding box across all
targets, although we were a bit faster with the bounding box
for small targets, there is a highly signiﬁcant difference between predictions and observed
times for small targets with a bounding box, but not with no bounding box. With no
• There is a signiﬁcant difference in movement time
between bounded and unbounded movements.
• This effect is only signiﬁcant for small targets.
• Instruct participants on how to approach the
target, in order to control for the effects of
• careful aiming versus quick movements
• We did not remove outliers, and our averages
may have been skewed by such points
What would we do differently?
★ Perform test on tablet with physical bounding
★ Add additional target sizes between small (20
pixels) and large (100 pixels) to ﬁnd out when
our effect becomes signiﬁcant.
★ Test for External Validity: Compare differences
in tab switching time between browsers
Chrome on Windows
Chrome on Mac OS