1. © Simeon Keates 2009 Usability with Project Lecture 14 – 30/10/09 Dr. Simeon Keates

2. © Simeon Keates 2009 Exercise – Part 1  Last week you were asked to prepare your user trial protocols  Today – put them into practice  Perform a pilot study of the usability of your web-site with at least 1 user  Remember – the principal aim is to “test the test” (or “trial the trial” or “evaluate the evaluation”…) Page 2

3. © Simeon Keates 2009 Exercise – Part 2  Prepare a progress presentation for the board for Friday  Show that good progress is being made  Summarise: The tasks performed The data collected Whether the user liked the site Whether the user could use the site (e.g. complete the tasks) What you think is working well in the design What you think needs to be looked at more closely in the design Any changes you would like to make to the site and protocol Page 3

4. © Simeon Keates 2009 Exercise - Practicalities  Remember to print out copies of your protocol  Allow plenty of blank space for adding observation notes  Allocate one person to do the pre-session briefing and debrief  Allocate one person to be the facilitator (the person who directs the user)  The remaining members act as observers Page 4

5. © Simeon Keates 2009 Cognitive models Page 5

6. © Simeon Keates 2009 The Power Law of Practice  T n = T 1 n - α  α = 0.4, T 1 = 60s, T 2 = 45.5s (24% faster), T 10 = 23.9s (60%faster) Page 6

7. © Simeon Keates 2009 Cognitive modelling – Dealing with uncertainty  The Uncertainty Principle states that decision time T increases with uncertainty about the decision to be made: T = I c H Where: H is the information-theoretic entropy of the decision; I c = 150 [0~157] ms/bit  For n equally probable alternatives (Hick’s Law) : H = log 2 (n + 1)  More generally: Page 7

8. © Simeon Keates 2009 Cognitive modelling – The Model Human Processor Time_taken = x τ p + y τ c + z τ m Where : x, y and z are integers τ p = time for perceptual processor τ c = time for cognitive processor τ m = time for (simple) motor function Page 8

9. © Simeon Keates 2009 Motor skills – Positioning time  The time to perceive something includes the time for your eye to be looking at the right thing  Similarly, motor functions also involve a “time for location”  Common sense says that: The further away something is, the longer it takes to reach it The smaller a target is, the longer it takes to “hit” it  Also, human movement is a 2 stage process  Stage 1 – gross (ballistic) movement Covers most of the distance quickly, but not very accurately  Stage 2 – fine (homing) movement Refine the position on to the target Page 9

10. © Simeon Keates 2009 Motor skills – Fitts’ Law  A person wishes to hit this target:  We know that a correction cycle takes: τ p + τ c + τ m ≈ 240 ms  And so n corrections takes n * 240 ms Page 10 Start x0x0 x1x1 x2x2 S D

11. © Simeon Keates 2009 Fitts’ Law  Now let x i be the remaining distance after the i-th correction  And let x 0 (= D) be the starting point  We will assume that the relative accuracy of movement is constant, i.e.:  Where ε < 1 and is the constant error  On 1 st cycle: x 1 = ε x 0 = ε D  On 2 nd cycle: x 2 = ε x 1 = ε (ε D) = ε 2 D  On n-th cycle: x n = ε n D  Process stops when: ε n D ≤ ½ S  Solving for n gives: Page 11

12. © Simeon Keates 2009 Fitts’ Law  From:  Total movement time, T pos is given by:  This can be re-written as: Where: ε has been found to be ~ 0.7 Thus I M ≈ -240 / log 2 (0.7) = 63 ms/bit [27~122 ms/bit] Page 12 Fitts’ Law

13. © Simeon Keates 2009 Fitts’ Law corrections  There are several modifications to Fitts’ Law  Fitt’s Law becomes less accurate for low values of log 2 (2D / S)  i.e. where the target is quite big compared with the distance  An example correction by Welford (1968): Page 13

14. © Simeon Keates 2009 Fitts’ Law – Implications for web-site design  Long, thin targets are not good Small S value => longer acquisition times  Example of long, thin target: Text-only hyperlinks e.g. Heinz tomato ketchup  Better to include something large e.g. an image of a ketchup bottle… Page 14

15. © Simeon Keates 2009 Merging the models One basic merged model is the Keystroke Level Model (KLM): T execute = T K + T P + T H + T D + T M + T R  Where T K = total time spent keystroking = n k t k (# * time per stroke) Time per stroke determined experimentally  T P = total time spent pointing (from Fitts’ Law) Assume, say, 1.1 s per pointing action  T H = total time spent homing (moving hands between devices) Assume 0.4 s per homing  T D = total time spent drawing = t D (n D, l D ) (i.e. f(#, total length)) Example: 0.9n D + 0.16l D  T M = total time to mentally prepare Assume 1.35 s per preparation  T R = total system response time Page 15

16. © Simeon Keates 2009 Using the KLM [Note: M = mental prep, K = keyboard, P = pointing]  Rule 0: Insert Ms in front of all Ks that are not part of argument strings proper. Place Ms in front of all Ps that select commands  Rule 1: If an operator following an M is fully anticipated in an operator just previous to M, then delete the M (e.g. PMK -> PK)  Rule 2: If a string of MKs belongs to a cognitive unit (e.g. name of a command), then delete all Ms but the first one  Rule 3: If a K is a redundant terminator (e.g. terminates a command immediately following the terminator of its argument), then delete the M in front of it  Rule 4: If a K terminates a constant string (e.g. a command name), then delete the M in front of it, but if the K terminates a variable string (e.g. an argument string) then keep the M in front of it Page 16

17. © Simeon Keates 2009 An more generic approach - GOMS The user’s cognitive structure consists of:  A set of Goals  A set of Operators  A set of Methods  A set of Selection rules Page 17

18. © Simeon Keates 2009 GOMS – a quick breakdown Goals:  Symbolic structures that define a state of affairs to be achieved Examples: GOAL: EDIT-MANUSCRIPT or GOAL: MODIFY-TEXT Goals can comprise sub-goals Operators:  Elementary perceptual, motor or cognitive acts whose execution is necessary to change any aspect of the user’s mental state or to affect the task environment Examples: GET-NEXT-PAGE or GET-NEXT-TASK Page 18

19. © Simeon Keates 2009 GOMS – a quick breakdown Methods:  Procedures for accomplishing a goal – must be pre-learned at performance time (i.e. user already knows them) Contain sets of Operators Selection rules:  Rules for helping the user decide which method to use to accomplish the goal Example: if_such_and_such_is_true_then_use_method_M1_else_use_M2 To summarise:  Several Operators make up a Method, and  Selection rules are used to determine the best Method to reach the Goal Page 19

20. © Simeon Keates 2009 Using models of interaction  Fundamentally, you need to perform a comprehensive task analysis  The models indicate suggested performance for each sub-task  Those models help you to predict the performance of the interface  This can be used: In design: Estimate performance using standard parameters to optimise your design In usability trials: Estimate the performance and compare with actual observed data – investigate significant discrepancies Page 20

21. © Simeon Keates 2009 Exercise Page 21

22. © Simeon Keates 2009 Exercise  On Wednesday(-ish) you performed a pilot study  Today, make any changes you identified to your usability protocol  Also, make any changes to your web-site based on the feedback that you obtained  Please mail your finalised protocols to Stina, Susanne and me Page 22