1. choose an iv 2. choose a dv 3.locate a convenient sample that is due to be exposed to a naturally...
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Non-Experimental Data:Natural Experiments
1. Choose an IV
2. Choose a DV
3. Locate a convenient sample that is due to be exposed to a naturally occurring stimulus (IV) (experimental group)
4. Locate a convenient sample that is NOT due to be exposed to that naturally occurring stimulus (IV) (control group)
5. Collect data on the DV from both populations (pre test).
6. Wait for the naturally occurring IV stimulus to affect the experimental group.
7. Collect data on your DV from both populations (post test).
8. Compare and analyse the two sets of results.
Natural Experiment (No control over experimental stimulus)
Natural/ ‘Quasi’ Experiments
Used to refer to situation that is not experimental but is ‘as if’ it was
Not a precise definition – saying your data is a ‘natural experiment’ makes it sound better
Refers to case where variation in X is ‘good variation’ (directly or indirectly via instrument)
A Famous Example: London, 1854
Snow- Cholera Epidemic
The Case of the Broad Street Pump
Regular cholera epidemics in 19th century London
Widely believed to be caused by ‘bad air’ John Snow thought ‘bad water’ was
cause Experimental design would be to
randomly give some people good water and some bad water
Ethical Problems with this
Soho Outbreak August/September 1854
People closest to Broad Street Pump most likely to die
But breathe same air so does not resolve air vs. water hypothesis
Nearby workhouse had own well and few deaths
Why is this a Natural experiment?
Variation in water supply ‘as if’ it had been randomly assigned – other factors (‘air’) held constant
Can then estimate treatment effect using difference in means
Or run regression of death on water source distance to pump, other factors
Strongly suggests water the cause Woman died in Hampstead, niece in
Islington
Other examples:
• Christianson and Hubinette (1993) Their experiment on anxiety
The influence of tv on aggressive behaviour(Charlton et al., 2000) (Williams, 1986)
TASK!
In pairs,Read the examples of natural experiments in your books on page 84 and answer 3.11