辅导案例-POLI 171
POLI 171: A Summary
Policy Making with Data
Two big questions to address
• Why do we need to use data in policy analysis and evaluation
• How do we use data?
Why do we do this?
Turns out͕ there is a lot of things ǁe don͛t knoǁ about the ǁorld
Why do we do this?
Turns out͕ there is a lot of things ǁe don͛t knoǁ about the ǁorld
͙and a lot of things ǁe thought we knew about the world
Oh ƚhe ƚhings ǁe don͛ƚ knoǁ
In the past, many terrible policies have been made with arguably good
intentions
• The United States recruited and sent people who measured below
mental and medical standards to Vietnam͕ hoping to ͞training and
opportunitLJ͟ to the uneducated and poor
• China exterminated rats, flies, mosquitoes, and sparrows
in an attempt to protect crops
• India and many countries instituted a ban on child labor
• Australia fought a war ʹ and lost ʹ against emus
Rigorous empirical research is the only way to subject our
beliefs and intentions to test
How we do this
The backbone of our analysis is the Potential Outcome Framework
How we do this
The backbone of our analysis is the Potential Outcome Framework
• The potential outcome model
• Causal effects
• The fundamental problem of causal inference
• Causal estimands: ATE, ATT
• Omitted variable bias
The potential outcome model
The potential outcome model
• For everLJ ͞treatment͟ ;a policLJ͕ membership in an
organization/community/group, a given characteristic, etc.), and for
every outcome, each observation has two potential outcomes
• An outcome under treatment condition (Y1)
• An outcome under control condition (or, in the absence of the treatment) (Y0)
• Which of the two outcome is exhibited depends on treatment status
• Let the observed outcome be Y
• If observation is treatedÆ we observe only treated outcome: Y=Y1
• If observation is untreated Æ we observe only untreated outcome: Y=Y0
Causal Effect
Fundamental Problem of Causal Inference
1. We can never observe both Y1i and Y0i simultaneously
2. As a result, we can never know causal effect with certainty
Causal Estimand: Average Treatment Effect
(ATE)
Average Treatment Effect on the Treated (ATT)
Omitted variable bias
Omitted variable bias
What is NOT omitted variable bias:
• Variables that influence likelihood of getting treatment but absolutely no
independent relationship with outcome
• e.g. A thunderstorm makes large-scale protests less likely to happen (treatment), but
;arguablLJͿ has no independent relationship ǁith federal government s͛ ǁillingness to
implement social change
• In practice, quite difficult to find example of things that truly have no independent
relationship with outcome
• Variables that influence outcome but have no relationship with treatment
• e.g. The amount of sleep is correlated with adult height (outcome), but has no
relationship with the amount of milk consumed during childhood
• Also similar: Variables in how treated observations take up a treatment
• e.g. Whether people wear masks correctly influence COVID-19 infection likelihood
(outcome), but does not influence likelihood of wearing mask (treatment)
Omitted variable bias
• Selection bias:
• A characteristic of an individual that makes them systematically more or less
likely to select themselves into the treatment condition AND exhibit
systematically different outcome
• e.g. Diligence. Diligent students are more likely to attend review session (the
treatment) and also tend to score higher in exams (the outcome)
• Endogeneity (aka reverse causality)
• Where an individual s͛ outcome influences their tendencLJ to get treatment
• e.g. Healthy people tend to eat well and engage in regular exercise, which in
turn improve health
Identification strategies
1.Experimental methods: Randomized Control Trials
2.Non-experimental methods
• Matching
• Regression
• Difference-in-Differences
Randomized experiments
• What are the stages of an experiment?
• What does random assignment do?
• How to estimate the treatment effect in an experiment?
• How to improve precision?
• Assumptions?
• Advanced designs?
Stages of an experiment
Random Assignment Prevents Omitted
Variable Bias
Estimation in randomized experiments
We use the difference in means estimator, and test for its statistical
significance using a t-test.
All of this are included in R through the lm() function:
݉݋݈݀݁ ൏ െ ݈݉ሺ~ݐݎ݁ܽݐ, ݀ܽݐܽሻ
ݏݑ݉݉ܽݎݕሺ݉݋݈݀݁ሻ
Accuracy vs. Precision
(Unbiasedness vs. Reliability)
How to increase precision:
Increase the size of our sample
• Higher sample -> law of large number kicks in -> lower impact of extreme outliers
Make our treatment group smaller than control group
• Technically reduces precision, but allows you to offer much bigger sample size given same
cost
Controlling for pre-treatment variables
• Reduce variations in outcome that͛s not caused bLJ variations in treatment status
Differencing our outcome variable
• Reduce variations in outcome that s͛ not caused bLJ variations in treatment status
Blocking on pre-treatment variables
• Increases similarity between treated and control group with regard to blocked variables
Clustering
• Actually decreases precision in exchange for less costly implementation AND reduce chance
of spillover effect
How to increase precision:
Precision is reflected in standard error
Standard error: The standard deviation of a sampling
distribution of an estimate
Lower precision -> Larger standard error compared to the
estimated treatment effect -> lower p-value
Assumptions
Excludability:
OnlLJ the treatments and nothing else outside the researcher s͛
control are ͞assigned͟ to the groups
Non-interference/No spillovers/SUTVA:
One unit s͛ treatment status should not influence another unit s͛
outcome
Assumptions can never be tested!
Advanced designs
Multiple treatment arms
• One group receives no treatment
• One group receives treatment A1
• One group receives treatment A1 + A2
• One group receives treatment Aϭ н AϮ н Aϯ͙
• Effect of each component estimated by comparing one group with the one
immediate to it
Factorial experiment (Interaction effects)
• One group receives no treatment
• One group receives treatment A
• One group receives treatment B
• One group receives treatment A + B
• Effect of interaction effect estimated by comparing A+B effect with sum of A s͛ and B͛s
effect
Non-experimental designs
When we do this?
• We have some treated and control units
• We didn͛t assign the treatment
Methods
• Matching
• Regression
• Diff-in-diff
What we covered
• Intuition
• Assumptions
• Code
Matching: Intuition
• For each treated unit, find one control/untreated unit that resembles
it the most in pre-treatment variables
• Discard all control observations that have no match
• Then, pretend we have an experiment and perform the same analysis
Matching: Assumptions
• Selection on observables:
• Whatever drives selection into treatment or control group have already been
observed and measured
• Two units that have the same observed pre-treatment variables have the
same likelihood of being in treated or control group.
• Their eventual treatment status is ͞as-if͟ random
Matching: Code
Matching and estimation performed through Match() function in Matching package
݉ܽݐ݄ܿ.݉݋݈݀݁ ൏ െܽݐ݄ܿሺ, ݎ, , ൌ 1, ݁ݔܽܿݐ ൌ , ݎ݁݌݈ܽܿ݁ ൌ ,
݁ݏݐ݅݉ܽ݊݀ ൌ "", ݅ܽݏ݆݀ݑݏݐ ൌ ሻ
ݏݑ݉݉ܽݎݕሺ݉ܽݐ݄ܿ.݉݋݈݀݁ሻ
Y A vector of outcomes. Example: df$outcome
Tr A vector of treatment status. Example: df$treat
X A vector of pre-treatment variables to match on. Example:
df΀͕c;͞age͕͟͟income͕͟͟educ͟Ϳ΁
M M matches per treated unit
exact Whether to do exact matching
replace Whether to reuse matched control units
estimand Which quantity to estimate.
BiasAdjust Whether to do extra regressions to adjust for remaining imbalances. Needs
replace=TRUE to work.
Matching: Code
• Exact matching: Set the argument exact=TRUE in Match() function
• Tips: Try to use only categorical or binary variables
• Distance matching: Set the argument exact=FALSE in Match() function
• Default is normalized Euclidean distance, which is somewhat similar to Mahalanobis
distance
• Propensity score matching
• Manually calculate propensity score:
model. ݌ݎ݋݌ ൏ െ ݈݉ ݐݎ݁ܽݐ~ݔ1 ൅ ݔ2 ൅ ݔ3, ݀ܽݐܽ ൌ ݂݀
݌ݎ݋݌ ൏ െ݉݋݈݀݁. ݌ݎ݋݌$݂݅ݐݐ݁݀. ݒ݈ܽݑ݁ݏ
• Then put the vector of fitted values into the argument X=prop in Match() function
match.model ൏ െܽݐ݄ܿሺ, ݎ, ൌ ݌ݎ݋݌, ൌ 1,
݁ݔܽܿݐ ൌ , ݎ݁݌݈ܽܿ݁ ൌ ,
݁ݏݐ݅݉ܽ݊݀ ൌ "", ݅ܽݏ݆݀ݑݏݐ ൌ ሻ
ݏݑ݉݉ܽݎݕሺ݉ܽݐ݄ܿ.݉݋݈݀݁ሻ
Matching: Code
Balance tests performed through MatchBalance() function
ܽݐ݄݈ܿܽܽ݊ܿ݁ ݂݋ݎ݉ݑ݈, ݀ܽݐܽ,݉ܽݐ݄ܿ. ݋ݑݐ
formul Treatment status variable on left, pre-treatment
variables on right. Example: treat~x1+x2+x3
data The dataset containing observations to match
match.out Output of a Match() function. Include when you
want to compare before vs. after match
Regression: Intuition
• Do not discard any unit
• Include all pre-treatment variables into a regression model, and take
advantage of its poǁer to statisticallLJ ͞hold everLJthing constant͟
• We consider the coefficient of the treatment variable our estimated
treatment effect
• It s͛ like magic͕ but cooler
Regression: Assumptions
• Selection on observables
• Linear relationships of variables on outcome
• A bunch of other assumptions about the standard errors
Regression: Code
Simply use the lm() function
݉݋݈݀݁ ൏ െ ݈݉ ~ݐݎ݁ܽݐ ൅ ݔ1 ൅ ݔ2 ൅ ݔ3… , ݀ܽݐܽ
ݏݑ݉݉ܽݎݕሺ݉݋݈݀݁ሻ
Regression: Code
If you include a categorical variable in the model, or convert a
numerical variable into categorical using as.factor(variable), R will
perform a fixed effects regression
• Do this when you suspect observations from different groups behave
differently in ways you cannot fully measure
• When reading regression outcomes, focus on estimated treatment
effect and standard error of the treatment ʹ don͛t ǁorrLJ too much
about the many estimates of the fixed effects
Difference in differences: Intuition
• Two groups, two time periods
• In first period, no group receives treatment
• In second period, one group receives treatment
• We measure ;ϭͿ hoǁ first group s͛ outcome changes betǁeen Ϯ
periods͕ and ;ϮͿ hoǁ second group s͛ outcome changes betǁeen Ϯ
periods
• Take the difference between (2) and (1) to find the treatment effect
Difference in differences: Assumptions
• Parallel trends: Outcomes of treated group would have moved the
same way as the outcome group in the absence of treatment
• Stable Composition: Groups have same membership over time
• ͞Nothing else happens͗͟ Treatment is the onlLJ thing that happens to one
group and not other after the treatment
Difference in differences: Code
Estimation is performed through lm() function
First, find out if your data is in the long or in the wide format
Long format:
݉݋݈݀݁ ൏ െ ݈݉ ~ݐݎ݁ܽݐ ൅ ݂ܽݐ݁ݎ ൅ ݐݎ݁ܽݐ ∗ ݂ܽݐ݁ݎ ൅ ݔ1 ൅ ݔ2, ݀ܽݐܽ
ݏݑ݉݉ܽݎݕሺ݉݋݈݀݁ሻ
treat whether observation comes from group that eventually
gets treatment
after whether observation is in post-treatment period
x1, x2 additional controls
Difference in differences: Code
Estimation is performed through lm() function
First, find out if your data is in the long or in the wide format
Wide format:
݂݀$݂݂݀݅ ൏ െ ݂݀$1 െ ݂݀$0
݉݋݈݀݁ ൏ െ ݈݉ ݂݂݀݅~ݐݎ݁ܽݐ ൅ ݔ1 ൅ ݔ2, ݀ܽݐܽ ൌ ݂݀
ݏݑ݉݉ܽݎݕሺ݉݋݈݀݁ሻ
treat whether observation comes from group that eventually
gets treatment
after whether observation is in post-treatment period
x1, x2 additional controls
Difference in differences: Code
Be aware that the standard errors of diff-in-diffs estimates are often
wrong
Solutions: Clustered standard errors, HC standard errors,
bootstrapping, etc.
How can I remember all of this?
The ansǁer͗ No͕ LJou can͛t
The ansǁer͗ No͕ LJou can͛t
͙ but that s͛ alright
Iƚ s͛ alrighƚ ƚo forgeƚ sƚƵff
Causal Inference
• You͛re gonna forget all the Y1, Y0 stuff
• But LJou͛ve seen hoǁ good research is done
Statistics
• You͛re gonna forget bias correction and clustered SEs
• But you know good statistical analysis is not scary
R
• You͛re gonna forget all the messy arguments or how to fix a for loop
• But hopefullLJ LJou͛re not afraid of ǁriting code anLJmore
Key take-aways
Correlation is not causation
• Mainly because of selection bias
Compare like with like
• Find methods to eliminate selection bias
Think of the counterfactuals
• Use statistics to predict counterfactuals
No substitution for good on-the-ground research
• Assumptions are edžamined through intense detective ǁork and ͞knoǁing the case͟
Seeks evidence to falsify your beliefs, not to confirm them
• Hypothesis testing matters in real life
What can you do with this knowledge
• Jobs in business analytics, government, or non-profit sector
• Data analysis
• Consulting
• Field research
What can you do with this knowledge
• Jobs in business analytics, government, or non-profit sector
• Data analysis
• Consulting
• Field research
• Bridge the ideological gap in debates on social issues
What can you do with this knowledge
• Jobs in business analytics, government, or non-profit sector
• Data analysis
• Consulting
• Field research
• Bridge the ideological gap in debates on social issues
• Support your causes
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