1. CS160 Discussion Section Fitts Law and KLM David Sun Sept 26 th 2007

2. 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.

3. Example:

4. 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.

5. More on Fitts Law Limitations: –One dimensional model of human movement –But often applied to two dimensional target acquisition tasks. D S start D S

6. More on Fitts Law The amplitude of the move and the width of the terminating region are measure along the same axis. D S start D S D D

7. Examples: D start S D S D S

8. Fixes Smaller-of: smaller of the two dimensions (since small side is more indicative of the task complexity). Computes a upper- bound. –Requires both dimensions and D –Simple but less accurate. Calculate W’ through trig functions. –Requires everything Smaller-of needs + angle of motion.

9. 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

10. 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

11. 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)

12. 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

13. Example