HCI 3e - Ch 17 (extra):

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additional slides for Chapter 17: Models of the system

from
Dix, Finlay, Abowd and Beale (2004).
Human-Computer Interaction, third edition.
Prentice Hall. ISBN 0-13-239864-8.
http://www.hcibook.com/e3/

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HCI 3e - Ch 17 (extra):

  1. 1. what is state <ul><li>that in the present </li></ul><ul><li>of that in the past </li></ul><ul><li>which affects that of the future </li></ul>time
  2. 2. defining state <ul><li>two problems: </li></ul><ul><li>too little state </li></ul><ul><ul><li>elements missing from specification </li></ul></ul><ul><ul><li>may be deliberate e.g. dialogue level spec. </li></ul></ul><ul><li>too much state </li></ul><ul><ul><li>too many states, too complex state </li></ul></ul><ul><ul><li>may be deliberate redundancy, extensibility </li></ul></ul>
  3. 3. too little state <ul><li>forgotten elements </li></ul><ul><ul><li>e.g. ‘typing’ flag for calculator </li></ul></ul><ul><li>checking: </li></ul><ul><ul><li>dialogue state can you work out current dialogue state? </li></ul></ul><ul><ul><li>action specification do you have enough information? </li></ul></ul><ul><ul><li>implicit global variables (see also later) suggest state missing </li></ul></ul>
  4. 4. too much state <ul><li>unreachable states </li></ul><ul><ul><li>too few actions (see later) </li></ul></ul><ul><ul><li>constraints </li></ul></ul><ul><li>spare variables: constant/functional dependent </li></ul><ul><li>dependent state </li></ul><ul><ul><li>e.g. first point of line, number being typed </li></ul></ul><ul><li>indistinguishable states </li></ul><ul><ul><li>what is observable? </li></ul></ul>states are not orthogonal
  5. 5. defining actions <ul><li>framing problems </li></ul><ul><ul><li>= too little in result state </li></ul></ul><ul><li>unreachable states – insufficient actions </li></ul><ul><li>using ‘global’ variables </li></ul><ul><ul><li>implicit in operation definition </li></ul></ul><ul><li>beware extreme cases </li></ul><ul><ul><li>(e.g. empty document, cursor at end of line) </li></ul></ul>

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