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 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

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 designs • Definition • Reasons for using within subjects designs

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

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

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

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

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

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

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 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

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

Order effects these result from the position in the sequence of treatments that a particular treatment has. Two types of stage of practice 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

Sequence effects These result from interactions among the treatments (also known as differential transfer effects). Two types of stage of practice 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

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

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

Within subjects designs • Remedies • Complete within subjects design • Block randomization • ABBA counterbalancing • Incomplete within subjects design

There are two approaches to arranging the order of treatments in a complete within subjects design. Block randomization ABBA counterbalancing Complete Within Subjects Designs

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

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

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

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

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.

Linear practice effect RT Trial #

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.

Non-linear practice effect RT Trial #

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

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

In this design: Hypothesis is tested within subjects. Practice effects are controlled between groups of subjects. Incomplete W.S. design – definition

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

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

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

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

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

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 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

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 • Grice & Hunter (1964) • Kahneman et al. (1993) • Lee & Katz (1998)

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)

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

Within subjects designs