Lecture 7: One-way and two-way repeated measures ANOVA Flashcards
Diagram of Repeated measures regression model
In regression model for repeated measures ANOVA, we have - (2)
a model for each participant with the values of u tweaking the model to account for individual differences in the baseline mean and the change in mean associated with the predictor(s)
g denotes the condition and i is the participant
One-way ANOVA between groups can be linked to linear model shown below..
- We have three means and the model accounts for three levels of a categorical variable with dummy variables
Diagram of repreated measure design equation
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Repeated-measures is a term used when the
same participants participate in all conditions of an experiment
What is the decision tree for choosing one-way repeated measures ANOVA? - (5)
Q: What sort of measurement? A: Continuous
Q:How many predictor variables? A: Two or more
Q: What type of predictor variable? A: Categorical
Q: How many levels of the categorical predictor? More than two
Q: Same or Different participants for each predictor level? A: Same
The assumption of sphericity in within-subject design ANOVA can be likened to
the assumption of homogeneity of variance in
between-group ANOVA
Sphericity is sometimes denoted as
ε or circularity
Sphericity is
a more general condition of
compound symmetry
What is compound symmetry?
true when both the variances across conditions are equal and the covariances between pairs of conditions are equal
Compound symmetry holds
true when both the variances across conditions are equal and the covariances between pairs of conditions are equal
In other words, it means..
variation within experimental conditions is fairly similar (similar to homogenity of variance in between) and that no two conditions are any more dependent than any other two
Sphereicty is a less restrictive form of
compound symmetry
What does spherecity refer to?
equality of variances of the differences between treatment levels.
Spherecity means the equality of variances of the differences between treatment levels
E.g.,if you were to take each pair of treatment levels, and calculate the differences between each pair of
scores, then it is
necessary that these differences have approximately equal variances.
you need at least … conditions for spherecity to be an issue
three
How is sphereicty assumed in this dataset?
How is spherecity calculated? - (2)
- Calculating differences between between pairs of scores for all treatment levels e.g., A-B, A-C , B-C
- Calculating variances of these differences e.g., variances of A-B, A-C, B-C
What does the data from table show in terms of assumption of spherecity (calculated by hand)? - (3)
there is some deviation from sphericity because the variance of the differences between conditions A and B (15.7) is greater than the variance of the differences
between A and C (10.3) and between B and C (10.3).
However, these data have local circularity (or local sphericity) because two of the variances of differences are identical.
The deviation from spherecity in the data does not seem too severe (all variances roughly equal) but here assess deviation is serve to warrant an action
How to assess the assumption of sphereicity in SPSS?
via Mauchly’s test
If Mauchly’s test statistic is significant (p < 0.05) then
variance of differences between conditions are significnatly different - must be vary of F-ratios produced by computer
If Mauchly’s test statistisc is non significant (p > 0.05) then it is reasonable to conclude that the
varainces of the differences between conditions are equal and does not significantly differ
Signifiance of Mauchly’s tes is dependent on
sample size
Example of signifiance of Maulchy’s test dependent on sample size - (2)
in big samples small deviations from sphericity can be
significant,
small samples large violations can be non-significant
What happens if the data violates the sphereicity assumption? - (2)
several corrections that can be applied to
produce a valid F-ratio
or
use multivariate test statistics (MANOVA)
What corrections to apply to produce valid F-ratio when data violates sphereicity? - (2)
- Greenhouse-Geisser correction ε
- Huynh-Feldt correction
The Greenhouse-Geisser correction varies between
1/k (k is number of repeated measures conditions) and 1
The closer that Greenhouse Geisser correction is to 1, the
more homogeneous the variances of differences, and hence the closer the data are to being spherical.
How to calculate lower-bound estimate fo spherecity for Greenhouse-Geisser correction when there is 5 conditions? - (2)
Limit of f ε^ is 1/k (number of repeated-measures conditions)
so… 1/(5-1) = 1/4 = 0.25
Huynh and Feldt (1976) reported that when the
Greenhouse-Geisser correction is too conservative
Huynh-Feldt correction is less conservative than
Greenhouse-Geisser correction
when estimates of sphericity are greater than 0.75 (1/k) then the
Huynh–Feldt
correction should be used,
when sphericity estimates are less than 0.75 (1/k) or nothing is
known about sphericity at all, then
the Greenhouse–Geisser correction should be used instead
Why is MANOVA used when data that violates spherecity?
MANOVA is not dependent upon the assumption of sphericity
In repeated measures ANOVA, the effect of our experiment is shown up in within participant variance than
between group variance
In independent ANOVA, the within-group variance is our…. and it is not contaimed by… - (2)
residual variance (SSR) = variance produced by individual differences in performance
SSR is not contaimined by experimental effect as study carried out by different people
In repeated-measures ANOVA, the within-participant variability is made up of
the effect of experimental manipulation and individual differences in performance (random factors outside of our control) - this is error SSR
Similar to independent ANOVA, repeated-measures ANOVA uses F-ratio to - (2)
compares the size of the variation due to our experimental
manipulations to the size of the variation due to random factors
has same type of variances in independent - total sum of squares (SST), model sum of squares (SSM) and a residual sum of squares (SSR)
What is the differences between independent ANOVA and repeated-measures design ANOVA?
repeated-measures ANOVA the model and residual sums of squares are both part of the within-participant variance.
In repeated-measures ANOVA
If the variance due to our manipulations is big relative to
the variation due to random factors, we get a .. and conclude - (2)
big value of F ratio
we can conclude that the observed results are unlikely to have occurred if there was no effect in the population.
To compute F-ratios we first compute the sum of squares which is the following… - (5)
- SST
- SSB
- SSW
- SSM
- SSR
How is SST calculated in one-way repeated measures ANOVA?
SST = grand variance (N-1)
What is the DF’s of SST?
N-1