1 within subjects designs 1. definition 2. reasons for using within subjects designs 3. stage of...

1

Within subjects designs

1. Definition

2. Reasons for using within subjects designs

3. Stage of Practice effectsA. Definition

B. Two types of practice effects Order effects Sequence effects

2


3. Stage of practice effects (continued)C. Remedies

• Complete W.S. design• Incomplete W.S. design

4. Limitations of within subjects designs5. Examples of W.S. designs

A. Grice & Hunter (1964)B. Kahneman et al. (1993)C. Lee & Katz (1998)

3


1. Definition

4

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.

5

Within subjects designs – definition

• 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

6


1. Definition


7



• Few subjects are available

• Increase efficiency• No acceptable

matching procedure• Increase sensitivity• Study differences in

subjects over time• Compare to between

groups design

8

Reasons for using the W.S. design

1. Few subjects are available

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

9



2. 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.

10




3. No acceptable matching procedure

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

11





4. 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

12





4. Increase sensitivity

5. 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)

13



2. Increase efficiency3. No acceptable

matching procedure4. Increase sensitivity5. Study differences in

subjects over time6. Compare to between

groups design

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

• E.g., Grice & Hunter (1964)

14


3. 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

15

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.

16





17

Two types of stage of practice effects

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

18

A B C D

A B C D

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

19

Two types of stage of practice effects

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

20

A B C D

C B A D

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?

21





22

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

23


• Complete within subjects design

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

24


• Incomplete within subjects design

• Subjects get each treatment only once.

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

25


3. Remedies– Complete within subjects design

• Block randomization• ABBA counterbalancing

– Incomplete within subjects design

26

Complete Within Subjects Designs

• There are two approaches to arranging the order of treatments in a complete within subjects design.

• Block randomization• ABBA

counterbalancing

27

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

28

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

29


• 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

30


• 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

Trial RT Practice 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.

32

Linear practice effect

Trial #

RT

Trial RT Practice 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.

34

Non-linear practice effect

Trial #

RT

35


• Incomplete within subjects designs

• Definition• All possible orders• Selected orders

– Latin square– Random starting order

with rotation

36

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

37

Incomplete W.S. design – definition

• In this design: • Hypothesis is tested within subjects.

• Practice effects are controlled between groups of subjects.

38

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.

39

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

• For 3 treatments (A, B, and C):

treatment order• Subj # 1st 2nd 3rd

• 1 A B C• 2 A C B• 3 B A C• 4 B C A• 5 C A B• 6 C B A

41

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.

42

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.

43

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

44

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

45

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).

46

Examples of within subjects designs

• Grice & Hunter (1964)

• Kahneman et al. (1993)

• Lee & Katz (1998)

47

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

48

Kahneman et al. (1993)

• Examined effects of three pain characteristics on the memory for pain.

• Duration of pain• Worst moment• Final moment

49


• 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

50


• 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

51


• 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

52


• 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?

53

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

54

Lee & Katz (1998)

• Example– “What a sunny day”

• Made on a rainy day – irony

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

55

Lee & Katz (1998)

• 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

56

Lee & Katz (1998)

• 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

57

Lee & Katz (1998)

Speaker as victim

Mean rating = 4.90

S.d. = 1.34

Listener as victim

Mean rating = 6.43

S.d. = 0.73

58

Lee & Katz (1998)

• 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

59

Lee & Katz (1998)

• 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 Listener conditions between groups would let group differences on sensitivity or probability of use affect means

1 within subjects designs 1. definition 2. reasons for using within subjects designs 3. stage of...

Documents