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CS160 Discussion Section Fitts Law and KLM

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

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CS160 Discussion Section Fitts Law and KLM

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  1. CS160 Discussion SectionFitts Law and KLM David Sun Sept 26th 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 D start start S S

  6. 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 S D start S

  7. Examples: D S start S D D start S start

  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

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