13.Significance Testing for Fixed Effects

Introduction to Mixed Models
Training session with Dr Helen Brown, Senior Statistician, at The Roslin Institute, March 2016.
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These training sessions were given to staff and research students at the Roslin Institute. The material is also used for the Animal Biosciences MSc course taught at the Institute.
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*Recommended UA-cam playback settings for the best viewing experience: 1080p HD
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Content:
Significance Testing for Fixed Effects
Base on ‘Wald’ F and t tests
Degrees of freedom (DF) for these tests should:
--- Correspond to the variance of contrast
--- Reflect all sources of variability
Balanced (or complete) data :
--- F is ratio of mean squares
--- Denominator DF then corresponds to DF of denominator mean square (often the residual)
Unbalanced data :
--- DF less straightforward to calculate
--- Satterthwaite’s approximation available
Hypertension Trial Example - Significance tests, SAS Output
Test of overall treatment equality :
Pairwise comparisons of treatments :
Denominator DF varies due to the differences in distribution of treatments across centres.