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Usability with Project Lecture 14 – 30/10/09

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

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Usability with Project Lecture 14 – 30/10/09

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  1. Usability with ProjectLecture 14 – 30/10/09 Dr. Simeon Keates

  2. 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”…)

  3. 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

  4. 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

  5. Cognitive models

  6. The Power Law of Practice • Tn = T1 n-α • α = 0.4, T1 = 60s, T2 = 45.5s (24% faster), T10 = 23.9s (60%faster)

  7. Cognitive modelling – Dealing with uncertainty • The Uncertainty Principle states that decision time T increases with uncertainty about the decision to be made: T = Ic H Where: H is the information-theoretic entropy of the decision; Ic = 150 [0~157] ms/bit • For n equally probable alternatives (Hick’s Law) : H = log2(n + 1) • More generally:

  8. Cognitive modelling – The Model Human Processor Time_taken = x τp + y τc + z τm Where : x, y and z are integers τp = time for perceptual processor τc = time for cognitive processor τm= time for (simple) motor function

  9. 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

  10. Motor skills – Fitts’ Law • A person wishes to hit this target: • We know that a correction cycle takes: τp + τc + τm≈ 240 ms • And so n corrections takes n * 240 ms S x0 x1 x2 Start D

  11. Fitts’ Law • Now let xi be the remaining distance after the i-th correction • And let x0 (= 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 1st cycle: x1 = ε x0 = ε D • On 2nd cycle: x2 = ε x1 = ε (ε D) = ε2 D • On n-th cycle: xn = εn D • Process stops when: εn D ≤ ½ S • Solving for n gives:

  12. Fitts’ Law • From: • Total movement time, Tpos is given by: • This can be re-written as: Where: ε has been found to be ~ 0.7 Thus IM ≈ -240 / log2(0.7) = 63 ms/bit[27~122 ms/bit] Fitts’ Law

  13. Fitts’ Law corrections • There are several modifications to Fitts’ Law • Fitt’s Law becomes less accurate for low values of log2(2D / S) • i.e. where the target is quite big compared with the distance • An example correction by Welford (1968):

  14. 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…

  15. Merging the models One basic merged model is the Keystroke Level Model (KLM): Texecute = TK + TP + TH + TD + TM + TR • Where TK = total time spent keystroking = nk tk (# * time per stroke) • Time per stroke determined experimentally • TP = total time spent pointing (from Fitts’ Law) • Assume, say, 1.1 s per pointing action • TH = total time spent homing (moving hands between devices) • Assume 0.4 s per homing • TD = total time spent drawing = tD (nD, lD) (i.e. f(#, total length)) • Example: 0.9nD + 0.16lD • TM = total time to mentally prepare • Assume 1.35 s per preparation • TR = total system response time

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. Exercise

  22. 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

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