Repeated Measures Designs Flashcards
What are the two sources of variation?
Systematic variation - due to the experimenter doing something to all of the participants in one condition
Unsystematic variation - variation from random factors that could not be controlled
Where does the source of variance come from?
Within the participants
Benefits
Sensitivity - unsystematic variance is reduced, more sensitivity to experimental effects - will see experimental effect more
Economy - less participants are needed, but fatigue, practise effects
What is the variance in a repeated measures design?
The variation in an individuals scores across the different conditions
small variance = results similar over conditions
big variance = results very different
What is sphericity?
Sphericity refers to the equality of variances of the differences between treatment levels - the relationship between pairs of pairs of groups is similar
the correlation across conditions should be the same
the variances of differences between conditions is equal
What assumption is violated?
Independent errors:
Because the same participants are in all conditions, scores are correlated with each other as it is the same persons scores
What does sphericity do?
Correct the degrees of freedom
What estimates are used?
Greenhouse-geisser estimate
Hyynh-feldt estimate
Lower bound estimate
What tests for sphericity?
Mauchly’s test - don’t do this
P less than .5 =violated
Bigger = met
When is sphericity met?
When the variances across all of the conditions are the same
How do you correct for the degrees of freedom?
Times the degrees of freedom by the amount of sphericity
eg. if GHG = .533
DOF = 3
3 times .533 = 1.59
these are your new degrees of freedom
What is perfect sphericity?
1
What follow up tests can you do?
Built in contrasts (simple contrast - dummy coding)
Post hoc tests - more limited options for this
What does bonferroni do?
The alpha rate never rises above 5%
a divided by number of tests
What is crucial in repeated measures designs?
Counterbalancing
Latins square - make an N by N square where each condition appears once in each row and column
3 conditions - 3x3 = 9 cells
