Lecture 8 Complex Experimental Designs PSY2004 Research Methods & Ethics in Psychology PSY2005 Applied Psychology Research Methods & Ethics

Experimental Designs Experiments Quasi-Experiments Characterised by random assignment of groups or participants to conditions. At least one variable is manipulated by the experimenter(s). Usually employs a control group Experiments Groups or subjects not randomly assigned (e.g., sample of convenience) May not have a comparison group Quasi-Experiments

The Principle of Analysis Of VAriance (ANOVA) Total Variability Systematic Variance (caused by IVs and found in differences between means Unsystematic variance (caused by things we do not know about)

Independent Variables Types of Variable what is manipulated in the experiment Independent Variables what is measured Dependent Variables what is held constant Control Variables what is allowed to vary randomly Random Variables correlated with an independent variable Confounding variable

Design Terminology Between Groups Repeated Measures Mixed An “independent groups” design Different groups Dependent measures taken one time Between Groups A “within groups” design One group, different conditions Dependent measures taken multiple times Repeated Measures Between and repeated Mixed

Single factor (one-way) Design Complexity Studies one independent variable Single factor (one-way) Studies multiple independent variables May have several levels Examples: Two-way (e.g., 2 x 2) Three-way (e.g., 2 x 2 x 2) Factorial

Factorial Design Two or more independent variables are studied in order to determine their independent and interactive effects on the dependent variable.

Unsystematic variance (caused by things we do not know about) The Principle of Analysis Of VAriance (ANOVA: mood by intelligence example) Total Variability Systematic Variance mood intelligence Interaction Unsystematic variance (caused by things we do not know about)

Main Effect Two or more independent variables are studied in order to determine their independent effects on the dependent variable.

Example Main Effect of Intelligence on Creativity Conclusion: people that are high in IQ score higher on creativity Tests regardless of mood. No effect for mood

Interaction Effect The effect of one factor (independent variable) depends on the level of the other factor (other independent variable).

Main Effects & Interactions High High Sadness Sadness High High Low Low Important point is that whenever both main effects and interaction effects are significant, only the interaction effect is interpreted since interaction effect modify the meaning of the main effect if it were to be interpreted on its own. Lightness Lightness Seasonal Affective Disorder No Seasonal Affective Disorder

A Detailed Example Godden and Baddeley (1975) Memory Context Study Participants (who were divers) learned lists of words either on the beach, or beneath 15 feet of water and then recall in the same or opposite environment. 2 IVs: Place of learning (2 levels: Beach or Underwater) Place of recall (2 levels: Beach or Underwater) DV: Number of words recalled

A Detailed Example Four Conditions: Learn on Beach Recall on Beach Learn Underwater Recall Underwater Recall on Beach

A Detailed Example

Advantages of Factorial Designs More than one hypothesis can be tested. Potentially confounding variables can be built into the design as factors. Enables interaction effects to be tested.

Randomised Controlled Trials An experimental design used to evaluate the effectiveness of psychological interventions. Participants assigned randomly, to one of at least two conditions Pre & post tests Outcomes measured before & after intervention Control group Includes no treatment; treatment as usual; and ‘wait list’ sample.

Example Does Psychological Therapy improve outcomes for People with Mild/Moderate Depression? Psychological Therapy (CBT + TAU) Sample Ramdomise Treatment as Usual TAU 2 IVs: 1) Time of measurement (pre- or post) 2) Treatment Group (CBT or TAU) DV: Self-reported mood/symptoms

Results of Study Main effect of Time? Yes Main effect of type of treatment? Possibly but not important Interaction between time and type of treatment? Yes Simple main effects? Conclusion: These results show a significant interaction between time and intervention. Whilst the participants reported lower depression following Treatment this effect was stronger in the CBT Treatment

Complex Designs 1 Complex designs represent research which has more than one independent variable. Complex designs have more than one outcome: main effects and interaction effects Example complex designs Two independent groups IVs and one DV Place of learning (beach/underwater): Main effect 1 Place of recall (beach/underwater): Main effect 2 Learning by recall: interaction effect DV: memory recall

Complex Designs 2: Independent Groups A independent groups designs is one in which the participant is in a single level of one IV. The role of attraction and type of crime in determining prison sentence IV1: Attraction: Attractive/Ordinary IV2: Type of Crime: Robbery/Swindle DV: Time spent in jail

The role of type of crime and attraction on determining prison sentence ME Attraction: NS ME Type of Crime: Swindle > Robbery A x TofC: Significant cross-over effect Conclusion: Although attractive people are likely to be given lenient sentences, when they use their attractiveness to commit a crime they are given longer sentences.

Complex Designs 2: Repeated Measures Design A repeated measures designs in which the participant takes part in all of the conditions in the experiment Can mood effect performance on visual and verbal forms of creativity Mood (positive/negative) Type of creativity (visual/verbal) DV: creativity task performance

Results Type of Creative: main effect fully explained by interaction Mood: main effect fully explained by interation T x M: Significant effect that explains the systematic variance found Conclusion: that people perform better on visual creativity tasks When they are in a positive mood.

Complex Designs: The mixed model A mixed model is one in which there is at least one independent groups IV and at least one repeated measures IV. Example: The effects of sex differences on types of intelligence Sex (male/female): IG variable Intelligence (spatial/verbal): RM variable Sex by type of intelligence: Interaction effect Intelligence score: DV

Outcome for the mixed model ME Intelligence: No effect ME sex: No effect I x S: Significant crossover effect Conclusion: both within and between women are better at verbal tasks and men are better at Spatial tasks