1. Motor Behaviour Models ISP666

2. Outline for today • Modelling • Descriptive models • Affordance • Bimanual behaviors • Break • Predictive models • Hick-Hyman law • Fitts's law • Power law of practice

3. Modelling Simplification of reality Useful Understanding complex phenomena Evaluating systems without tests Designing new systems There's nothing so practical as a good theory. - Kurt Lewin

4. Kinds of Model Descriptive models Framework Categories Identifiable features Often verbal or diagrammatic (not precise) Predictive models Can make verifiable predictions Often mathematical (precise) Not many in HCI

5. Affordances Affordance: action possibilities an artefact offers to a user Other examples?

6. Affordance • Gibson's theory • Ecological approach • Affordance is a relationship • neither an objective property nor a subjective property • Don Norman's book • The Design of Everyday Things • Perceived vs real affordance

7. Norman's Principle for Screen Design 1. Follow conventional usage, both in the choice of images and the allowable interactions. 2. Use words to describe the desired action if possible 3. Use metaphor. 4. Follow a coherent conceptual model so that once part of the interface is learned, the same principles apply to other parts. Discussion: How to apply them? Can you give examples?

8. Guiard's Bimanual Model Nonpreferred Hand leads the preferred hand sets the spatial frame of reference for the preferred hand performs coarse movements Preferred Hand follows the non-preferred hand works within established frame of reference set by the non-preferred hand performs fine movements

9. Desktop Design Implications (accidental) left-hand bias of current desktop Left hand – mouse, fine movement Right hand – keyboard, coarse movement

10. Desktop Design Implications Better scrolling design (for right handed people) Left hand – scrolling Set the frame of reference Coarse movement

11. Human as Information Processor • Information pass through various channels • From outside world • Among different cognitive processing modules • To outside world

12. Information Theory Shannon-Weaver model General model of communication A way to quantify information

13. Information Information is defined as reduction in uncertainty Amount of uncertainty is called entropy bits n: is number of alternatives pi: probability of ith alternative

14. Hick's Experiments Attempt to apply information theory to psychological problems Simple reaction time over multiple choices

15. Hick-Hyman Law Linear relationship between RT and entropy

16. Implication of Hick-Hyman Speed-accuracy trade-off More information is process in accurate mode than in speed mode Stimulus-response compatibility (SRC) Better SRC has higher information transfer Novice cognitive processes Doesn't work on well practiced processes

17. Fitts' Experiment Movement over distance to reach target Movement time as function of target width and movement amplitude

18. ID: index of difficulty (bits) Determined by Amplitude / Width Time is linear relationship to ID Fitts' Law

19. Implication of Fitts' Law Speed-accuracy trade-off Comparison of physical devices Throughput (TP) = IDe / T Where IDe is computed with effective target width (We = c x Sdx) Used extensively in studying all kinds of movement: Pointing, navigation, text entry, head, foot movement, etc. Very robust

20. Power Law of Practice Learning and practice follow a power law, i.e. people improve in speed at a decaying rate. Tn = T1 n-a ,a ~ 0.4 Tn : the time to perform a task after n trials. T1 : the time to perform a task in the first trial. n : the number of trials

21. Limitation of Models Over simplification Premature closure May lead to confusion of model and reality

22. Food volunteer?