Lecture bioinformatic databases - Wednesday - Mixed models
12 important questions on Lecture bioinformatic databases - Wednesday - Mixed models
What is wide format?
When to use wide or long format?
Long = easier if unequal number of measurements per person
How to change from wide to long format in SPSS (or vice versa)?
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What is important for residuals? (ANOVA and linear regression)
What if residual is not distributed normally / not independent?
What is a cluster?
--> weight of puppies is more similar within litter
How many dummy variables do you need?
What is the difference between random and fixed effects?
Random effects are the effects of the dummy variables you may add (abbreviated to bi in the equation). A high variance for the dummy variable, means that correction was needed. You compare the variance of the dummy variable with the residual, dummy > residuals means that correction was correct
What if you ignore the non-independence of the residuals?
When to use a random effect and when a fixed effect?
But when there are a lot of clusters (also with countries), you should use random effects. Because the cluster effects would be different when repeating the study. With a few countries (3) the cluster effects are expected to be the same, then it is a fixed effect
Sometimes mixed models are too complicated, what is then an admissible solution?
Advantages:
- simple analysis
- easy to interpret
- might be more robust
Disadvantages:
- information might be lost (power decreases, but not always)
- sometimes difficult to reduce
What do you need to include in the equation when time trends differ between groups (different slopes)?
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