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Within subjects designs Definition Reasons for using within subjects designs Stage of Practice effects Definition Two types of practice effects Order effects Sequence effects Within subjects designs Stage of practice effects (continued) Remedies Complete W.S. design

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within subjects designs
Within subjects designs
  • Definition
  • Reasons for using within subjects designs
  • Stage of Practice effects
    • Definition
    • Two types of practice effects
      • Order effects
      • Sequence effects
within subjects designs2
Within subjects designs
  • Stage of practice effects (continued)
    • Remedies
      • Complete W.S. design
      • Incomplete W.S. design
  • Limitations of within subjects designs
  • Examples of W.S. designs
    • Grice & Hunter (1964)
    • Kahneman et al. (1993)
    • Lee & Katz (1998)
within subjects designs definition
When a variable is manipulated within subjects, all subjects receive all levels of that variable.

A given study can use only between groups variables, only within-subjects variables, or a combination of the two.

Within subjects designs – definition
within subjects designs definition5
For example, suppose you want to know which of three kinds of car is most comfortable to drive on a long journey.

You have a Ford, a Chevy, and a Toyota, and 10 drivers (the subjects)

Each driver drives each car on the same length journey and rates each for comfort

Within subjects designs – definition
within subjects designs6
Within subjects designs
  • Definition
  • Reasons for using within subjects designs
within subjects designs7
Reasons for using within subjects designs

Few subjects are available

Increase efficiency

No acceptable matching procedure

Increase sensitivity

Study differences in subjects over time

Compare to between groups design

Within subjects designs
reasons for using the w s design
Few subjects are available

E.g., research with patients with particular impairments that are important but uncommon, such as deep dyslexia or prosopagnosia

Reasons for using the W.S. design
reasons for using the w s design9
Few subjects are available

Increase efficiency

Answer more questions with the same number of subjects

E.g., instead of dividing 40 subjects among two treatment groups for one study, use them in two separate studies.

Reasons for using the W.S. design
reasons for using the w s design10
Few subjects are available

Increase efficiency

No acceptable matching procedure

For example, if you cannot measure enthusiasm, speed of processing, efficiency of attention, etc.

Reasons for using the W.S. design
reasons for using the w s design11
Few subjects are available

Increase efficiency

No acceptable matching procedure

Increase sensitivity

Sensitivity refers to the ability to detect differences in performance produced by the treatment

Analogous to turning up the magnification of a microscope

Reasons for using the W.S. design
reasons for using the w s design12
Few subjects are available

Increase efficiency

No acceptable matching procedure

Increase sensitivity

Study differences in subjects over time

Learning

Psychophysics

Whenever you want subjects to compare two or more stimuli relative to one another

E.g., Kahneman et al. (1993)

Reasons for using the W.S. design
reasons for using the w s design13
Few subjects are available

Increase efficiency

No acceptable matching procedure

Increase sensitivity

Study differences in subjects over time

Compare to between groups design

Treatment might have different effect in within subjects vs. between groups designs.

E.g., Grice & Hunter (1964)

Reasons for using the W.S. design
within subjects designs14
Within subjects designs
  • Stage of practice effects
    • Definition
    • Two types of stage of practice effects
      • Order effects
      • Sequence effects
    • Remedies
      • Complete within subjects design
      • Incomplete within subjects design
stage of practice effects definition
The changes subjects undergo with repeated testing are called stage of practice effects.

With repeated testing, subjects’ performance on a task may get:

better if a skill is being developed;

worse if fatigue or boredom increase.

Stage of Practice Effects – Definition
within subjects designs16
Within subjects designs
  • Stage of practice effects
    • Definition
    • Two types of stage of practice effects
      • Order effects
      • Sequence effects
    • Remedies
      • Complete within subjects design
      • Incomplete within subjects design
two types of stage of practice effects
Order effects

these result from the position in the sequence of treatments that a particular treatment has.

Two types of stage of practice effects
order effects
If B and D give different results, is that treatment effect?

Subjects might just be more tired, or more skilled, when they get D

Order effects

A B C D

A B C D

two types of stage of practice effects19
Sequence effects

These result from interactions among the treatments (also known as differential transfer effects).

Two types of stage of practice effects
sequence effects
B follows A vs. B follows C

This difference could produce sequence effects – is a B / C difference due to treatment or due to what they follow?

Sequence effects

A B C D

C B A D

within subjects designs21
Within subjects designs
  • Stage of practice effects
    • Definition
    • Two types of stage of practice effects
      • Order effects
      • Sequence effects
    • Remedies
      • Complete within subjects design
      • Incomplete within subjects design
stage of practice effects remedies
Before considering remedies, we have to distinguish between two types of W.S. design:

Complete within subjects design

Incomplete within subjects design

Stage of Practice effects – Remedies
stage of practice effects remedies23
Complete within subjects design

Subjects get each treatment often enough to balance stage of practice effects for each subject.

Stage of Practice effects – Remedies
stage of practice effects remedies24
Incomplete within subjects design

Subjects get each treatment only once.

Levels of I.V. are confounded with order levels are presented in

Stage of Practice effects – Remedies
within subjects designs25
Within subjects designs
  • Remedies
    • Complete within subjects design
      • Block randomization
      • ABBA counterbalancing
    • Incomplete within subjects design
complete within subjects designs
There are two approaches to arranging the order of treatments in a complete within subjects design.

Block randomization

ABBA counterbalancing

Complete Within Subjects Designs
block randomization
Each block of trials contains one trial for each treatment.

Number of blocks = number of times each treatment is administered.

Order of treatments randomized within a block

Works better with many trials per treatment

Block randomization
abba counterbalancing
In general, counterbalancing controls for practice effects by presenting the treatments in multiple sequences

ABBA Counter-balancing presents treatments in a sequence, then presents them in the reverse sequence.

Repeat as often as needed to generate desired amount of data per treatment

ABBA counterbalancing
abba counterbalancing29
Can be used with any # of treatments and repeated any # of times within an experiment

For 3 treatments, use ABCCBA, etc.

Must repeat whole sequence, not just a part of it

ABBA counterbalancing
abba counterbalancing30
Anticipation effects may be a problem, especially if there are many cycles through the sequence.

Works well when practice effects are linear.

Does not work with non-linear practice effects. For non-linear effects, stabilize performance with practice trials before recording data

ABBA counterbalancing
slide31

TrialRTPractice effect

1 550 --

2 525 25

3 500 25

4 475 25

5 450 25

6 425 25

This shows a linear practice effect – increase in speed is the same every trial. ABBA counter-balancing works in this case.

slide33

TrialRTPractice effect

1 550 --

2 500 50

3 470 30

4 460 10

5 455 5

6 453 2

This shows a nonlinear practice effect – increase in speed is larger in the early trials. ABBA counterbalancing is no help in this case.

within subjects designs35
Incomplete within subjects designs

Definition

All possible orders

Selected orders

Latin square

Random starting order with rotation

Within subjects designs
incomplete w s design definition
Each subject gets each treatment once.

Practice effects are balanced across subjects rather than within subjects.

Levels of the I.V. are confounded with order of presentation within any subject

Thus data for individual subjects are not interpretable

Incomplete W.S. design – definition
incomplete w s design definition37
In this design:

Hypothesis is tested within subjects.

Practice effects are controlled between groups of subjects.

Incomplete W.S. design – definition
incomplete w s design
General rule for these designs:

Each treatment condition must appear in each ordinal position of the sequence equally often.

The techniques that follow vary in what additional counter-balancing effects they achieve, but all achieve this effect, so all produce interpretable data.

Incomplete W.S. design
all possible orders
Preferred incomplete W.S. design technique

All treatments appear in each ordinal position equally often.

Each treatment precedes & follows every other one equally often at each ordinal position

All possible orders
slide40
For 3 treatments (A, B, and C):

treatment order

  • Subj #1st2nd3rd
  • 1 A B C
  • 2 A C B
  • 3 B A C
  • 4 B C A
  • 5 C A B
  • 6 C B A
selected orders
We often have 5 or more treatments in one study.

5 treatments = 120 possible orders.

6 treatments = 720 possible orders.

Too many subjects!

When we have many treatments, we use selected orders.

That is, from the set of all possible orders we use only a subset.

Selected orders
selected orders latin square
Each treatment appears equally often at each ordinal position

Each treatment precedes & follow every other treatment exactly once

Limited to experiments with an even number of treatments

Procedures for creating Latin Squares appear in advanced texts.

Selected orders – Latin square
selected orders random starting order with rotation
Start with any order

With each new subject, rotate each treatment one position to the left in the sequence

each condition appears in each ordinal position an equal number of times

but each condition precedes & follows same conditions throughout

advantages: simplicity, applicability

Selected orders – random starting order with rotation
random starting order with rotation example with four treatments
Random starting order with rotation – example with four treatments

Subj # Treatment order

1 D A C B

2 A C B D

3 C B D A

4 B D A C

5 D A C B

limitations of w s designs
W.S. designs cannot be used:

On subject variables such as age and sex.

With unfolding sequences of successive events (for example, animal in Operation condition cannot also be in Anesthesia-only condition).

If each treatment takes a long time (e.g., 1 year).

Limitations of W.S. designs
examples of within subjects designs
Examples of within subjects designs
  • Grice & Hunter (1964)
  • Kahneman et al. (1993)
  • Lee & Katz (1998)
grice hunter 1964
Classical conditioning study

Two different intensities of sound as C.S.s

In general, a more intense C.S. gives stronger classical conditioning

G & H found stronger effect of sound intensity in a within subjects version of the study than in a between groups version

Grice & Hunter (1964)
kahneman et al 199349
Condition A

Subject keeps hand in 14° C water for 60 seconds

Condition B

Subject keeps hand in water for 90 seconds

60 seconds at 14° C plus 30 extra seconds during which temperature rises gradually to 15° C

Kahneman et al. (1993)
kahneman et al 199350
One trial per condition

Half of subjects got A first then B

Half of subjects got B first then A

7 minute distracter task

Subjects asked which condition they preferred to repeat

60% chose B

Kahneman et al. (1993)
kahneman et al 199351
D.V. was choice of pain.

You can only use this D.V. with a within-subjects design

Subjects must get both conditions if they are to choose between them

Kahneman et al. (1993)
kahneman et al 199352
You could do this experiment with a different D.V. – say, pain ratings – which would allow a between groups design

But would groups be comparable?

More ‘sissies’ in one group than the other?

Kahneman et al. (1993)
lee katz 1998
Study of figurative language

Distinguished between irony and sarcasm

Both ‘figures’ involve saying something you know is not true

Lee & Katz: sarcasm has a victim; irony does not

Lee & Katz (1998)
lee katz 199854
Example

“What a sunny day”

Made on a rainy day – irony

Made on a rainy day to someone who predicted sunshine – sarcasm

Lee & Katz (1998)
lee katz 199855
Manipulation:

Subjects read eight passages and rate each for sarcasm on a 7-point scale

Two I.V.s manipulated within subjects: prediction and victim identity

We’ll look at victim identity today

Lee & Katz (1998)
lee katz 199856
Prediction

A prediction made in the passage was either true or false

E.g., prediction that it will be a sunny day

Victim identity

Either the speaker or the listener

E.g., either the speaker or the listener had predicted sunshine

Lee & Katz (1998)
lee katz 199857
Speaker as victim

Mean rating = 4.90

S.d. = 1.34

Listener as victim

Mean rating = 6.43

S.d. = 0.73

Lee & Katz (1998)
lee katz 199858
Same passage was rated as a better example of sarcasm when listener was the victim

Why? Perhaps because people don’t usually make sarcastic remarks about themselves

Lee & Katz (1998)
lee katz 199859
Subjects are expressing an opinion – is a remark sarcastic?

They may vary in sensitivity to sarcasm or the probability they would use sarcasm

Comparing rated sarcasm for Speaker and Listenerconditions between groups would let group differences on sensitivity or probability of use affect means

Lee & Katz (1998)