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Analysis of propensity-score matched data
HHS Methods Meeting 2012 Felix Thoemmes
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Propensity scores and regression
1. Propensity scores are a principled approach to causal inference, regression adjustment is not
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(𝑌0,𝑌1 ) ⊥ 𝑇|𝑋 Ignorability
Propensity scores and regression
1. Propensity scores are a principled approach to causal inference, regression adjustment is not
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Both rely on same assumption of no unmeasured confounders
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Propensity scores and regression
2. Propensity scores are outcome-free and are not victim to (unintentional) biases
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Propensity scores and regression
2. Propensity scores are outcome-free and are not victim to (unintentional) biases
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In ideal study in which outcomes and covariates are clearly separated avoidance of bias is possible
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Propensity scores and regression
3. Functional form assumptions of outcome relationship are more restrictive
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Propensity scores and regression
3. Functional form assumptions of outcome relationship are more restrictive
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Regression models do not have to be modeled linearly. Linearity violations can to some degree be assessed. Progress with data mining techniques.
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Propensity scores and regression
4. Regression extrapolates, whereas propensity scores put focus on common support
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Propensity scores and regression
4. Regression extrapolates, whereas propensity scores put focus on common support
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Common support in multivariate space can be assessed without propensity scores (convex hull).
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Propensity scores and regression
Regression adjustment
≈ Propensity score matching
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Effects of military training
• Big Five personality traits are conducive to change
• Important life events can impact personality development
• Jackson, J., Thoemmes, F., Jonkmann, K., Lüdtke, O., & Trautwein, U. (2012). Military training and personality trait development: Does the military make the man or does the man make the military? Psychological Science.
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Effects of military training
12 Photo Credit: Stock Photograph by Department of Defense Public Domain. Copyright: United States Government Works
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Effects of military training
• Until very recently, 18 year old men in Germany had to do either civil service or military training
• Any impact of military training on personality that is observed is likely confounded by selection effects
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Effects of military training
• Large sample (~2,000) of college-bound youth were equated using PS matching on 38 covariates
• Pre-tests • Socio-economic status & cultural capital • Academic achievement • Vocational interests • …
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Selection effects (Differences t1)
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Effects after matching (Differences t2)
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Effects after adjustment (Differences t2)
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Longitudinal effects of military training
18 Latent growth model after propensity score matching
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Summary
• Effects of military training on agreeableness are present after 1 year and even after 4 years, as assessed in latent growth model
• Potential unobserved confounders ?
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Summary
• In this particular example regression adjustment ≈ propensity score matching
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