1 Within subjects designs Definition 2 Reasons for

1 Within subjects designs Definition 2. Reasons for using within subjects designs 3. Stage of Practice effects 1. Definition B. Two types of practice effects l Order effects l Sequence effects A.

2 Within subjects designs 3. Stage of practice effects (continued) C. Remedies • Complete W. S. design • Incomplete W. S. design Limitations of within subjects designs 5. Examples of W. S. designs 4. A. Grice & Hunter (1964) B. Kahneman et al. (1993) C. Lee & Katz (1998)

3 Within subjects designs 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 withinsubjects variables, or a combination of the two.

5 Within subjects designs – definition • For example, suppose • You have a Ford, a you want to know Chevy, and a Toyota, which of three kinds and 10 drivers (the of car is most subjects) comfortable to drive • Each driver drives on a long journey. each car on the same length journey and rates each for comfort

6 Within subjects designs 1. Definition 2. Reasons for using within subjects designs

7 Within subjects designs 2. 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

8 Reasons for using the W. S. design Few subjects are available 1. • E. g. , research with patients with particular impairments that are important but uncommon, such as deep dyslexia or prosopagnosia

9 Reasons for using the W. S. design Few subjects are available 2. Increase efficiency 1. • 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 Reasons for using the W. S. design Few subjects are available 2. Increase efficiency 3. No acceptable matching procedure 1. • For example, if you cannot measure enthusiasm, speed of processing, efficiency of attention, etc.

11 Reasons for using the W. S. design Few subjects are available 2. Increase efficiency 3. No acceptable matching procedure 4. Increase sensitivity 1. • Sensitivity refers to the ability to detect differences in performance produced by the treatment • Analogous to turning up the magnification of a microscope

12 Reasons for using the W. S. design 1. 2. 3. 4. 5. 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) •

13 Reasons for using the W. S. design 1. 2. 3. 4. 5. 6. 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)

14 Within subjects designs 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 Within subjects designs 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

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 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 A B C 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 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? A B C D C B A D

21 Within subjects designs 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

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 Stage of Practice effects – Remedies • Complete within subjects design • Subjects get each treatment often enough to balance stage of practice effects for each subject.

24 Stage of Practice effects – Remedies • Incomplete within subjects design • Subjects get each treatment only once. • Levels of I. V. are confounded with order levels are presented in

25 Within subjects designs 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 Counterbalancing 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 ABBA counterbalancing • 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 ABBA counterbalancing • 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 counterbalancing works in this case.

32 Linear practice effect RT Trial #

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 RT Trial #

35 Within subjects designs • Incomplete within subjects designs • Definition • All possible orders • Selected orders – Latin square – Random starting order with rotation

36 Incomplete W. S. design – definition • Levels of the I. V. are treatment once. confounded with order of presentation within • Practice effects are any subject balanced across subjects rather than within subjects. • Thus data for individual subjects are not interpretable • Each subject gets each

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 counterbalancing 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 # 1 st 2 nd 3 rd • • • 1 2 3 4 5 6 A A B B C C B C A 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.

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 43

Random starting order with rotation – example with four treatments Subj # Treatment order 1 2 3 4 5 D A C B D A C B 44

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 • G & H found stronger study effect of sound intensity in a within • Two different subjects version of intensities of sound as the study than in a C. S. s between groups • In general, a more version intense C. S. gives stronger classical conditioning

48 Kahneman et al. (1993) • Examined effects of three pain characteristics on the memory for pain. • Duration of pain • Worst moment • Final moment

49 Kahneman et al. (1993) • 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 Kahneman et al. (1993) • 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 Kahneman et al. (1993) • D. V. was choice of pain. • You can only use this D. V. with a withinsubjects design • Subjects must get both conditions if they are to choose between them

52 Kahneman et al. (1993) • 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: • Two I. V. s manipulated within subjects: • Subjects read eight prediction and victim passages and rate identity each for sarcasm on a 7 -point scale • 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 Listener as victim Mean rating = 4. 90 S. d. = 1. 34 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