Week 5 - PS Matching Flashcards
What is the difference between the common support requirements for matching for estimating the ATT and the ATE?
Estimating the ATT only requires the distribution of propensity scores for the treated is contained within the distribution of the untreated. ATE needs common support for both treated and control.
Why is it advantageous to match based on the logit of the propensity score rather than the propensity score itself?
Doing so avoids compression around zero and one.
How is greedy matching performed?
Choose a treated case and search for the best available match among the untreated cases without accounting for the quality of the match of the entire treated sample.
How is optimal matching performed?
Optimal matching uses a network flow optimization problem to minimize total weighted sample distance, thereby selecting the “optimal” match.
How is genetic matching performed?
Genetic matching uses a genetic search algorithm to find a set of weights for each covariate such that a version of optimal balance is achieved after matching.
What is the role of the generalized Mahalanobis distance in genetic matching?
The distance minimized by the genetic matching algorithm is the generalized Mahalanobis distance (GMD).
In what situation would greedy matching be expected to perform as well as optimal matching?
In situations when there are large numbers of control units available for matching to treated units.
What is the advantage of optimal matching over greedy matching?
Optimal matching performs as well or better than greedy matching with respect to minimum distance and balance.
What is the advantage of genetic matching over optimal and greedy matching?
Genetic matching attempts to minimize a global distance between the matches on all covariates.
What is the expected difference in performance between one-to-one, one-to-many, and variable ratio matching?
Greater bias reduction. One-to-many (never use), because you have to sacrifice quality to maintain the strict number assignment. One-to-one and variable ratio provide good quality matches.
How is a caliper used in matching and what is the advantage of using it?
Maximum distance to allow matches, and if no match is found within the distance the treated observation is dropped for lack of common support. It forces common support, and match quality.
How can common support be strictly enforced in propensity score matching?
Through the use of a caliper.
What is the expected difference in performance between matching with and without replacement?
Matching with replacement has the advantage of always matching each treated unit to the closest untreated unit, and therefore produces larger bias reduction than matching without replacement.
How is full matching performed?
Each treated unit is matched to at least one control unit and vice versa, without replacement. This procedure can be viewed as a propensity score stratification where the number of strata containing at least one treated and one untreated observation is maximized.
Which matching methods require the use of weights and why?
Genetic AND optimal matching… to optimize post-matching covariate balance.
