Concepts to be included Experimental group Control group Random assignment Posttest Pretest Treatment Observation

The classical experiment Henk van der Kolk

Aim Introducing the classical experiment: a research design for testing bivariate causal relationship. Also known as “Randomized experiment”.

Example Does watching television news affect the importance people attach to political topics?

Example Political opinions Watching news about a political topic Importance attached to that political topic

(Classical) Experiment (def.) A research design, in which... ...comparable groups of units are constructed using random assignment and... ...in which the treatment (independent variable) is manipulated differently across groups... ...to see whether the outcome (dependent variable) becomes different across groups. Deze slide is lastig om overzichtelijk te maken. Er staan nu steeds drie puntjes, maar die kun je gewoon weer weghalen als je het niks vindt.

Random assignment = randomization Experimental group Units Control group Random assignment

Randomization created Comparable groups Experimental group Pre-test Dep var Units = Control group Pre-test Dep var All other variables: gender, age, etc. Random assignment

Example Experimental group Important? Medium Units Medium Control group Important? All other variables: gender, age, etc.

The classical experiment Experimental group Pre-test Dep var Treatment Units Control group Pre-test Dep var Placebo Random assignment

Example Experimental group Important? News Units Medium Medium No news Control group Important? Random assignment

The classical experiment Experimental group Pre-test Dep var Treatment Post-test Dep var Units Compare Control group Pre-test Dep var Post-test Dep var Placebo Random assignment

Example Experimental group Important? News Important? Units Medium High Medium Medium No news Control group Important? Important? Random assignment

Three aspects of causality Time order; X precedes Y in time Association/correlation; X and Y are correlated Non spuriousness; there is no other (third) variable accounting for the correlation

Time order Because of the pre-test and the observed similarity between the two groups … Because we focus on the difference between the two groups AFTER giving the treatment … … we are sure the independent variable precedes the dependent variable: no reverse causation.

Example Experimental group Important? News Important? Units Medium High Medium Medium No news Control group Important? Important? Random assignment Time Is it possible that “importance” affects “watching the news”?

Association By treating one group and not the other By comparing the outcomes of the control group and the experimental group We establish (the absence of) an association.

Example Experimental group Important? News Important? Units Medium High Compare Medium Medium No news Control group Important? Important? We can check that those watching the news, attached more importance to a topic after watching the news

Non-spurious Because of random assignment and checked by the pre-test, the only difference between the control group and the experimental group is … … the threatment. We thus make sure, there is no effect of other (third) variables

Example Political opinions Watching news about a political topic Importance attached to that political topic

Classical experiments Two groups Randomization (= random assignment) One pre test of the dependent variable One post test of the dependent variable One intervention / treatment (the independent variable)

This microlecture Introducing the classical experiment as a research design for testing bivariate causal relationships Experimental group and Control group Random assignment Posttest and Pretest Treatment

Images used: Slide 5: https://pixabay.com/en/screen-television-silhouettes-310714/ Slide 8: https://www.flickr.com/photos/jeffanddayna/4081090389