Fit`s Law – Movement Time
Fitts's Law is basically an empirical model explaining speed-accuracy tradeoff characteristics of human muscle movement
MT represents the movement time to hit the target
a and b are empirically determined constants
A is the movement distance from the starting position to the target center
W is the target width
W is the target width
The empirical constants a and b are
found using a regression analysis on the movement time data.
Apply Fitts` law to
physical input device : MT is movement time , a is the intercept ( start/stop
time of the device ) , b is the slop ( inherent speed ) , A is target size , W
is height of button .
The first game is move a stylus alternately back and forth
between two separate target regions, tapping one or the other target at the end
of each movement , These types of tasks are called continuous tasks where the
subject is not expected to stop after finishing one movement but instructed to
repeat the same task symmetrically as quickly as possible. In continuous tasks,
the total time is divided by the number of movements to determine the average
movement time for a particular target size and distance.
So we choose different devices (
calculator / mobile phone / smart phone ) , we get three group of data , the
first group : 7+5 with calculator is best tool ; the second group : 7+3+1 with
Iphone is best tool; the third group : 4*9÷2 with Iphone is best tool. It`s
said that the basic pointing task variables A and W , (A/W+1) is the index
of difficulty .
The second game is discrete
tasks where the subject is stoped after one movement and the time is measured
between the start and the end-points. subjects should make their aimed
movements as rapidly as possible .
The
first group is one person hold the rule , the other one try to hold it when it
start fall by visual / auditory / tactile , the result is visual and auditory
almost have the same speed , but tactile need muscle transfer to neurons then
to control muscle to catch the rule . the second group is one person hold two
rules , the other one try to hold one of them , fall rule is random , so the
other one should judge which one he/she see is falling , then to catch it , the
result is visual use less time than auditory .
The third game is use testing tool
: use equipment to record time and tasks .one person put a button to turn on a
red light , the other one push another button to turn of the light , the
equipment record the reaction time .
From all of the test , it
prove that pointing speed, target distance, target size and accuracy are the
most influence of the movement time .
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