Single-Factor Experimental Designs

Brian J. Piper, Ph.D.

Goals

• Single Factor: 2 levels– Independent groups– Matched groups– Non-equivalent groups– Repeated measures (2)

• Single Factor: 3+ levels• Control Group Designs

Really?

• “Single factor studies using only two levels are not as common as you might think. Most researchers prefer to use more complex designs, which produce more elaborate and intriguing outcomes.”

2 Independent Groups

• Independent variable (IV) manipulated• Equivalent groups created by randomization• Does insecurity increase materialism?– Participants: undergraduate students (N=60)– IV: • Control: write essay about listening to music• Experimental: write essay about own death

Kasser & Sheldon (2000) Psychological Science, 11, 348-351.

t-test

• Student’s (Between Subjects) t-testt = (Mean1 – Mean2)/Variability12 if SD1 ≈SD2

Degrees of Freedom = N - 2

O

William S. Gosset

1876-1937

Assumptions of t-test1) Data is interval or ratio2) Data is normally distributed3) For two-groups, homogeneity of variance

Logic

If p > .05 assumption is metIf p < .05 than assumption not met.

Overview

t = Signal / Noise

+ ∞ to - ∞

2 Independent Groups

• Dependent Variable (DV): economic value

Mean (SD)t (degrees of freedom) = # , p <> .05

Matched Groups Example

• Some variable is measured & then group assignment

• Sleep deprivation & suggestibility example– Sleep length recorded before assigning to

deprivation (+ or - )– Long-story presented following by leading questions

(Were assailants armed with gun or knife?)

Blagrove (1996) J Exp Psychol: Applied, 2, 48-59.

Matched Groups Example

• Some variable is measured & then group assignment

• Sleep deprivation & suggestibility example• Results: 43, but not 21, hour deprivation

increased suggestibility

Blagrove (1996) J Exp Psychol: Applied, 2, 48-59.

Non-Equivalent Groups

• He quit because his “body forgot the urge to smoke”

• Comparison of brain damage to insula (N=19) versus damage to other areas (N=50)

Comparing Groups

• Smoking following brain damage was examined in patients with or without insula damage.

Naqvi et al. (2007). Science, 315, 531-534.

Results

Naqvi et al. (2007). Science, 315, 531-534.

Repeated Measures

• If same subjects get both treatments, there is a concern about order effects (fatigue, practice, etc.)

Repeated Measures

• If same subjects get both treatments, there is a concern about order effects (fatigue, practice, etc.)

• Solution: Counterbalance!

Mental Effects of Steroids

• Men (N=56) received:– steroids, placebo– Placebo, steroids

Pope et al. (2000) Archives General Psychiatry, 57, 133-140.

Repeated Measures: Example 2

• Do toddlers use visual feedback for balance or only mechanical feedback?

Lee & Aronson (1974) Perception & Psychophysics, 15, 529-532.

Repeated Measures Design

• Within-Subjects: order alternating– Forward – Backward – Forward – Backward– Backward – Forward – Backward - Forward

Lee & Aronson (1974) Perception & Psychophysics, 15, 529-532.

Repeated Measures Design

• Within-Subjects: order alternating– Forward – Backward – Forward – Backward– Backward – Forward – Backward - Forward

Lee & Aronson (1974) Perception & Psychophysics, 15, 529-532.

Paired t-test• Comparison of Novel Faces & Places Response

Times (msec) for correct versus incorrect decisions

• Degrees of Freedom = N - 2

2-Level Design Statistic

Independent groups Two-sample t-testMatched groups Paired t-testNon-equivalent groups Two-sample t-testRepeated measures (2) Paired t-test

3+ Levels

• Provide more information than 2 levels

3+ Levels

• Provide more information than 2 levels

1850-1909

Hermann Ebbinghaus

Data Presentation

• Whenever possible figures • If you must …………..tables

3+ Groups Analysis

• Analysis of Variance (ANOVA)– Signal / noise– Range: 0 to infinity, ≈3.0+ is statistically significant

1

ANOVA

• Step 1: Is there a difference (somewhere)?

• Degrees of freedom:– between groups = # groups -1– Error = total N – dfBG -1

• Step 2: Where is the difference?– T-test– Post-hoc tests (Tukey HSD)

1915-2000

Control Group Designs

• Placebo Group (aka negative control)– Alcohol flavored Jello shots– THC free cigarettes

• Wait List Control Group: depression example

Control Example

• Women (N=47) were randomly assigned to:– Subliminal: tapes consistent with labeling– Placebo: tapes inconsistent with labeling– Wait List: received tapes at end of 6 week study

Mirikle & Skanes (1992) J Applied Psychology, 77, 772-776.

Yoked Control• Participant groups matched based on prior trauma

(moderate), sex, and age• Eye Movement Desensitization & Reprocessing: imagine trauma + follow therapists

rapidly moving finger• Control: imagine trauma while staring at stationary object (same duration as

experimental

**

Dunn et al. (1996). J Behav Therapy Exp Psychiatry, 27, 231-239.

Yoked Control

• Part I:– Experimental: no barrier, light than jump– Yoked: barrier, gets same # of shocks

• Part II (no barrier)– Experimental: learned to avoid footshock– Yoked: learned to not avoid footshock

Summaryt-test ANOVA

assumptions Interval/ratio, normal, homogeneity of variance

Interval/ratio, normal, homogeneity of variance

groups 2 3+

statistic t, p < .05 F, p < .05

variations between, within between, within