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250b 4.4. effect modification 4.5. logistic regression 4.3.2. controlling confounding stratification 4.3.1. controlling confounding matching 4.2.4. confounding counterfactual 4.2.3. confounding collapsability 4.2.2. confounding classical approach 4.2.1. confounding mixing of effects 4.1. introduction 4 threats to validity from confounding bias and ef 6.7 summaries 6.6 examples 6.5 strengths and challenges 6.4.4 designs case-crossover 6.4.3 designs cumulative 6.4.2 designs case cohort 6.4.1 designs density 6.3 sources of controls 6.2 principles 6.1 big picture 2.5 summary 2.4.3 properties of scale ande other measures 2.4.2 absolute measures 2.4.1 relative measures 2.3 2x2 table 2.2 scales of measures 2.1 big picture 3.5.2 selection bias 3.6 comparing biases 3.7 preventing biases 3.5.1 information bias 3.5 types of biases 3.4 types of validity 3.3 hierarchy of populations 3.2 sourced of error 3.1 big picture