Mini_UPA, 2009 Rating Scales: What the Research Says Joe DumasTom Tullis UX ConsultantFidelity Investments

Mini_UPA, The Scope of the Session u Discussion of literature about rating scales in usability methods, primarily usability testing u Brief review of recommendations from older literature u Focus on recent studies u Recommendations for practitioners

Mini_UPA, Table of Contents u Types of rating scales u Guidelines from past studies u How to evaluate a rating scale u Guidelines from recent studies u Additional advantages of rating scales

Types of Rating Scales

Mini_UPA, Formats u One question format u Before-after format u Multiple question format

Mini_UPA, One Question Formats u Original Likert scale format: I think that I would like to use this system frequently: ___ Strongly Disagree ___ Disagree ___ Neither agree not disagree ___ Agree ___ Strongly Agree Rensis Likert

Mini_UPA, One Question Formats u Likert-like scales: Characters on the screen are: Hard to readEasy to read

Mini_UPA, One Question Formats One more Likert-like scale (used in SUMI): I would recommend this software to my colleagues: I would recommend this software to my colleagues: __ Agree__ Undecided__ Disagree

Mini_UPA, One Question Formats SubjectiveMentalEffortScale(SMEQ)

Mini_UPA, One Question Formats u Semantic Differential: u Magnitude estimation: u Use any positive number

Mini_UPA, Before-After Ratings u Before the task: How easy or difficult do you expect this task to be: Very easyVery difficult u After the task: How easy or difficult was task to do: Very easyVery difficult

Mini_UPA, Multiple Question Formats (Selected List) u Software Usability Scale (SUS) – 10 ratings u *Questionnaire for User-Interface Satisfaction – QUIS 71 (long form), 26 (short form) ratings u *Software Usability Measurement Inventory (SUMI) – 50 ratings u After Scenario Questionnaire (ASQ) – three ratings * Requires a license

Mini_UPA, More Multiple Question Formats u Post Study System Usability Questionnaire (PSSOQ) - 19 ratings. Electronic version called the Computer System Usability Questionnaire (CSUQ) u *Website Analysis and MeasureMent Inventory (WAMMI) – 20 ratings of website usability * Requires a license

Guidelines from Past Studies

Mini_UPA, Guidelines u Have 5-9 levels in a rating u You gain no additional information by having more than 10 levels u Include a neutral point in the middle of the scale u Otherwise you lose information by forcing some participants to take sides u People from some Asian cultures are more likely to choose the midpoint

Mini_UPA, Guidelines u Use positive integers as numbers u 1-7 instead of -3 to +3 (Participants are less likely to go below 0 than they are to use 1-3) u Or don’t show numbers at all u Use word labels for at least the end points. u Hard to create labels for every point beyond 5 levels u Having labels on the end points only also makes the data more “interval-like”.

Mini_UPA, Guidelines u Most word labels produce a bipolar scale u In a 1 to 7 scale from easy to difficult, what is increasing with the numbers? Is ease the absence of difficulty? u This may be one reason why participants are reluctant to move to the difficult end – it is a different concept that lack of ease u One solution – scale from “not at all easy” to “very easy”

Evaluating a Rating Scale

Mini_UPA, Statistical Criteria u Is it valid? Does it measure what it’s suppose to measure? u For example, does it correlate with other usability measures. u Is it sensitive? u Can it discriminate between tasks or products with small samples

Mini_UPA, Practical Criteria u Is it easy for the participant to understand and use? u Do they get what it means? u Is it easy for the tester to present – online or paper – and score? u Do you need a widget to present it? u Can scoring be done automatically?

Guidelines from Recent Studies

Mini_UPA, Post-Task Ratings u The simpler the better u Tedesco and Tullis found this format the most sensitive Overall this task was: Very Easy Very Difficult u Sauro and Dumas found SMEQ just as sensitive as Likert

Mini_UPA, More on Post-Task Ratings u They provide diagnostic information about usability issues with tasks u They correlate moderately well with other measures especially time, and their correlations are higher than for post-test ratings

Mini_UPA, More on Post-Task Ratings Even post-task ratings may be inflated (Teague et al., 2001). Ratings made during a task were significantly lower than after the task and even higher if given only after the task Concurrent During task Concurrent After task Post-taskOnly Ease

Mini_UPA, Post-Test Ratings u Home grown questionnaires perform more poorly than standardized ones u Tullis and Stetson and others have found SUS most sensitive. Many testers are using it. u Some of the standardized questionnaires have industry norms to compare against - SUMI and WAMMI u But no one knows what the database of norms contains

Mini_UPA, More on Post-Test Ratings u The lowest correlations among all measures used in testing are with post-task ratings (Sauro and Lewis) u Why? – they are tapping into factors that don’t effect other measures such as demand characteristics, need to please, need to appear competent, lack of understanding of what an “overall” rating means, etc.

Mini_UPA, Examine the Distribution See how the average would miss how bimodal the distribution is. Some participants find it very hard to use. Why? See how the average would miss how bimodal the distribution is. Some participants find it very hard to use. Why?

Mini_UPA, Low Sensitivity with Small Samples u Three recent studies have all shown that post-task and post-test ratings do not discriminate well with sample sizes below about u For sample sizes typical of laboratory formative tests, ratings are not reliable u Ratings can be used as an opportunity to get participant to talk about why they have chosen a value

Mini_UPA, The Value of Confidence Intervals Actual data from an online study comparing the NASA & Wikipedia sites for finding info on the Apollo space program.

Little Known Advantages of Rating Scales

Mini_UPA, Ratings Can Help Prioritize Work “Fix it Fast” “Promote It” “Big Opportunity” “Don’t Touch It” 1=Difficult … 7=Easy

Mini_UPA, Ratings Can Help Identify “Disconnects” This “disconnect” between the accuracy and task ease ratings is worrisome– it indicates users didn’t realize they were screwing up on Task 2!

Mini_UPA, Ratings Can Help You Make Comparisons You can be very pleased if you get an average SUS score of 83 (which is the 94 th percentile of this distribution). But you should be worried if you get an average SUS score of 48 (the 12 th percentile).

Mini_UPA, In Closing… u These slides, a bibliography of readings, and associated examples, can be downloaded from: Feel free to contact us with questions!