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Controlled User studiesHCI - 4163/6610Winter 2013

+Usability Experiments

Predict the relationship between two or more variables.

Independent variable is manipulated by the researcher.

Dependent variable depends on the independent variable.

Typical experimental designs have one or two independent variable.

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+True Experiment

Experimental control

Control as many potential threats to validity as possible

Random assignment of participants/data to conditions

Could be within-subjects or between-subjects

+ControlTrue experiment = complete control

over the subject assignment to conditions and the presentation of conditions to subjects Control over the who, what, when, where,

how

Control of the who => random assignment to conditions Only by chance can other variables be

confounded with IV

Control of the what/when/where/how => control over the way the experiment is conducted

+Quasi-Experiment

When you can’t achieve complete controlLack of complete control over

conditionsSubjects for different conditions come

from potentially non-random pre-existing groups (smokers vs nonsmokers)

+It’s a matter of control

True Experiment

Random assignment of subjects to condition

Manipulate the IV

Control allows ruling out of alternative hypotheses

Quasi Experiment Selection of subjects for

the conditions

Observe categories of subjects If the subject variable is

the IV, it’s a quasi experiment

Don’t know whether differences are caused by the IV or differences in the subjects

+Other features

In some instances cannot completely control the what, when, where, and howNeed to collect data at a certain time or not at

allPractical limitations to data collection,

experimental protocol

+Validity

Internal validity is reduced due to the presence of controlled/confounded variables But not necessarily invalid

It’s important for the researcher to evaluate the likelihood that there are alternative hypotheses for observed differences Need to convince self and audience of the validity

+External validity

If the experimental setting more closely replicates the setting of interest, external validity can be higher than a true experiment run in a controlled lab setting

Often comes down to what is most important for the research questionControl or ecological validity?

+TerminologyFactors: Independent Variables (Ivs) of an

experiment

Level: particular value of an IV

Condition: a group or treatment (technique) e.g., Condition 1: old system, Condition 2: new

system

Treatment: a condition of an experiment

Subject: participant (can also think more broadly of data sets that are ‘subjected’ to a treatment)

+Factors to Treatments

At least 1 Factor (IV) has to vary to have an experiment Effect of screen size and input technique on

performance (speed, accuracy)

An IV must always have at least 2 levels

Condition refers to a particular way that subjects are treated Between subject: experimental conditions are the

same as the groups Within subjects: only 1 group, that experiences every

condition (can be many conditions in an experiment)

+Good Experimental Design

Two-Group, Post-Test Design

Two conditions

Two groups: Between subjects: random allocation

Treatment

Post-test: measure the DV

What’s really important?

+Experimental designs

Between subjects: Different participants - single group of participants is allocated randomly to the experimental conditions.

Within subjects: Same participants - all participants appear in both conditions.

Matched participants - participants are matched in pairs, e.g., based on expertise, gender, etc.

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+Within-subjectsSimilar to the one-group pre-test-post-

test design

It solves the individual differences issues

But raises other problems: Need to look at the impact of experiencing

the two conditions Will they get tired? Gain practice? Learn

what is expected?

Need to control for order and sequence effects?

+Order Effects

Changes in performance resulting from (ordinal) position in which a condition appears in an experiment (always first?)

Arises from warm-up, learning, fatigue, etc.

Effect can be averaged and removed if all possible orders are presented in the experiment and there has been random assignment to orders

+Sequence effects

Changes in performance resulting from interactions among conditions (e.g., if done first, condition 1 has an impact on performance in condition 2)

Effects viewed may not be main effects of the IV, but interaction effects

Can be controlled by arranging each condition to follow every other condition equally often

+Counterbalancing

Controlling order and sequence effects by arranging subjects to experience the various conditions (levels of the IV) in different orders

Self-directed learning: investigate the different counterbalancing methods Randomization Block Randomization Reverse counter-balancing Latin squares and Greco squares (when you can’t fully

counterbalance) http://www.experiment-resources.com/counterbalanced-meas

ures-design.html

+ Between, within, matched participant design

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+ Key points 1· Usability testing is done in controlled conditions.

· Usability testing is an adapted form of experimentation.

· Experiments aim to test hypotheses by manipulating certain variables while keeping others constant.

· The experimenter controls the independent variable(s) but not the dependent variable(s).

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