using repeated measures in statistical analysis · 2017-04-05 · using repeated measures in...
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Using Repeated Measures in Statistical AnalysisMcKenzie Kuhn, Ben Bourrie, & Deanna Pepin
Pseudoreplication in ecological studies
Simple (subsamples) Temporal (repeated measures)
A1
A2
A1 A2 A3 A4
A2
A3
A4
B1
B2
B3
B4B1 B2 B3 B4
TIME
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Benefits of Repeated Measures
More statistical power
Fewer subjects
The solution: multilevel mixed effects modeling
Litter A
Cage1 Cage 2
Litter B
Cage3 Cage 4
Fixed Factor
Random
Bird 1
T1 T2 T3
Bird 2
T1 T2 T3
Bird 3
T1 T2 T3
Bird 4
T1 T2 T3
Bird 5
T1 T2 T3
Bird 6
T1 T2 T3
Bird 7
T1 T2 T3
Bird 8
T1 T2 T3
Bird 9
T1 T2 T3
Bird 10
T1 T2 T3
Bird 11
T1 T2 T3
Bird 12
T1 T2 T3
Random
Goal: look at effects of litter type and time (day) on weight gain
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• Missing Data:
• Multivariate approach: Sample dropped
• Mixed approach: Time point dropped
• Post hoc tests:
Why Use a Mixed Effect Model?
• Post hoc tests:
• Repeated measures
• Mixed approach
Important R packages
nmle, car, multcomp
Limitations
Data must be normal & group sizes equal
No method for multivariate repeated measures (yet!)
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Resources for pseudoreplication and mixed models
Davies, G. M., & Gray, A. (2015). Don't let spurious accusations of pseudoreplication limit our ability to learn from natural experiments (and other messy kinds of ecological monitoring). Ecology and Evolution, 5(22), 5295-5304.
Hurlbert, S. H. (1984). Pseudoreplication and the design of ecological field Hurlbert, S. H. (1984). Pseudoreplication and the design of ecological field experiments. Ecological monographs, 54(2), 187-211.
Zuur, A. F., Ieno, E. N., Walker, N. J., Saveliev, A. A., & Smith, G. M. (2009). Mixed effects models and extensions in ecology with R. Gail M, Krickeberg
K, Samet JM, Tsiatis A, Wong W, editors. New York, NY: Spring Science and Business Media.
http://rpsychologist.com/r-guide-longitudinal-lme-lmer
Thanks!