Experimental Designs

• Experimental design - definition:

  • In which one (or more) variable(s) is manipulated and the effect of this manipulation is observed in other variables
  • It aims to control all other variables
  • It allows us to infer causality
• Causality:

  • If there is change to A does a change in B result?
  • Cause must precede the effect
  • The cause and effect must co-vary
  • If the cause does not occur then neither does the effect
• Group designs - within or between subjects:

  • Within Group Design
  • One group of participants receives all experimental conditions (including control)
  • Offers paired data
  • Between Group Design
  • Different groups receive the different experimental conditions
  • Offers unpaired data
• Testing treatments: Specific and non-specific effects:

  • specific:
  • The effect of the proposed “active ingredient” or mechanism of the therapy
  • non-specific:
  • Natural history
  • Regression to the mean
  • Placebo / interaction effects
  • Resentful demoralisation/“frustrebo”/ nocebo
• • when we compare an active treatment to no intervention, we call that a pragmatic trial - where the active ingredient and non-specific interactions can explain the differences
  • when we compare an active treatment to a sham intervention, we call that an efficacy trial - where the active ingredient can explain the difference
  • when we compare no intervention to a sham intervention, the differences can be explained by non-specific effects

  
• Experimenting on the individual: Single subject designs/n of 1 studies:

  • tests intervention and its withdrawal, can allow randomisation, clinically achievable
  • A-B design
  • test the intervention on a patient, then withdraw the intervention to see if symptoms reverts - shows if the intervention causes the benefit, or if its natural recovery
  • A-B-A
  • test the intervention on a patient, withdraw the intervention, then give intervention again
  • A-B-A-B
  • A-B1-A-B2-A