CS160 Discussion Section
Fitts Law and KLM
David Sun
Sept 26th 2007
Fitts Law (recap)
• A model for computing the time it takes
to move the hand/pointer to a target.
• Formula: a + b log (D/S +1)
– D: distance to the target
– S: width of the target measured along the
dimension of motion.
– a, b: constants determined through
regression.
Example:
More on Fitts Law
• A number of formulations exist. We use the
Shannon Formulation (by Mackenzie).
– Slightly better fit to data
– Keeps ID positive
– Fits with underlying information theory
• Index of difficulty (ID): log (D/S +1), measured in
bits.
• Index of performance (IP) : 1/b, measured in
seconds/bit.
• IP is independent of the particular target.
More on Fitts Law
• Limitations:
– One dimensional model of human movement
– But often applied to two dimensional target
acquisition tasks.
D
D
start
start
S
S
More on Fitts Law
• The amplitude of the move and the width
of the terminating region are measure
along the same axis.
start
D
D
D
start
start
D
S
start
S
Examples:
D
S
start
S
D
D
S
start
start
Fixes
• Smaller-of: smaller of the two dimensions
(since small side is more indicative of the
task complexity). Computes a upperbound.
– Requires both dimensions and D
– Simple but less accurate.
• Calculate W’ through trig functions.
– Requires everything Smaller-of needs + angle
of motion.
Keystroke Level Model
• Describe the task using the following Operators
– K: pressing a key or a pressing (or releasing) of a button
• T(K) = 0.08~1.2 seconds (~0.2 avg)
– P: pointing
• T(P) = 1.1 seconds (without button presses)
– H: homing (switching device
• T(H) = 0.4 sec
– D(n,L): drawing segmented lines
• T(D) = 0.9n + 0.16L
– M: mentally prepare
• T(M) = 1.35s
– R(t) : system repsonse time
• T(R) = t
KLM Heuristic Rules (Raskin)
0: Insert M
–
–
in front of all K
in front of all P’s selecting a command (not in front of P’s ending a command)
1: Remove M between fully anticipated operators
–
MPK  PK
2: if a string of MKs belong to a cognitive unit, delete all M’s except the first
–
4564.23: MKMKMKMKMKMKMK  MKKKKKKK
3: if K is a redundant terminator, then delete M in front of it
–
[enter] [enter]: MKMK  MKK
4a: if K terminates a constant string (command name) delete the M in front of
it
–
cd [enter]: MKKMK  MKKK
4b: if K terminates a variable string (parameter) keep the M in front of it
–
cd class [enter]: MKKKMKKKKMK  MKKKMKKKKKMK
More on KLM
• Basic underlying cognitive assumption:
– Serial stage model of human information
processing: one activity is done at a time
until a task is complement.
– No parallel activities, no interruptions and
interleaving goals.
• Others models: NGOMSL (overlapping
human activities), CPM-GOMS (more
rigor)
Example:
K: pressing a key or pressing and releasing a button = 0.2s
P: pointing = 1.1s (without button press)
H: Homing (switching device) = 0.4s
M: Mentally prepare = 1.35s
Example