The goal of experimental research?
demonstrate changes in DV CAUSED by different levels of IV
Examples of demonstrating changes in DV caused by diff. levels of IV?
A drug treatment caused decreased depression
Training caused improved reading ability
Bystanders caused people to not offer help
Intellectual engagement caused successful aging
Practice caused people to use different parts of their brain to perform a task
If a causal relationship exists
DV should vary based on IV
Variability in scores caused by extraneous variables or participant variability
We are essentially searching for a signal (________) within noise (________)
the effect we are looking for; all the other things that might cause scores to change
How do we know differences in score (variability) is related to our IV?
total variance = systematic variance + error variance
Total variance = ?
Systematic variance = ?
Effect of IV
Error variance = ?
Effect of all other things
4 Basic Strategies for dealing with error variance?
Reduce Error Variance
Increase effectiveness of IV
Randomize Error Variance Across Groups
Reduce Error Variance?
Treat all participants the same
Match participants on variables that have an effect on DV
Increase Effectiveness of IV?
Use levels of IV that are very different
E.g., Effect of Alcohol on Memory
Sober vs .05 BAC
Sober vs .10 BAC
Randomize Error Variance Across Groups?
Participants have an equal chance of being in any group
Error variance between groups will tend to balance out
We've run an experiment and average scores are different across treatments
Participants treated for depression have a lower score on a depression index compared to untreated participants
How do we know that this difference wasn't there to begin with (e.g., due to Error Variance)?
What is the probability that the size of the effect we observed happened just by chance (as a result of error variance)?
Stats programs give us a p-value
5% chance typically considered acceptable
Between Participant Designs?
Different groups of participants are exposed to different levels of the independent variable
Within Participant Designs?
All participants are exposed to all levels of the independent variable