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Coarsened Exact Matching

Source: R/matching_cem.R
cem_match.Rd

Coarsens continuous variables into bins, then performs exact matching on the coarsened values. Units in strata containing both left and right units are kept; others are pruned. Matched units receive weights inversely proportional to stratum sizes to maintain balance.

Usage

cem_match(
  left,
  right,
  vars,
  cutpoints = NULL,
  n_bins = "sturges",
  grouping = NULL,
  keep = "all",
  left_id = "id",
  right_id = "id"
)

Arguments

left

Data frame of left (treated) units

right

Data frame of right (control) units

vars

Character vector of variable names to coarsen and match on

cutpoints

Named list of break vectors per variable. If NULL, automatic binning is used.

n_bins

Binning method when cutpoints is NULL: "sturges" (default), "fd" (Freedman-Diaconis), "scott", or an integer specifying the number of bins for all variables.

grouping

Character vector of variable names to match exactly (without coarsening). These are typically categorical variables.

keep

Which units to return: "all" (default) returns all units with weight 0 for unmatched, "matched" drops unmatched units.

left_id

Name of ID column in left (default: "id")

right_id

Name of ID column in right (default: "id")

Value

An S3 object of class c("cem_result", "couplr_result") containing:

matched

Tibble with columns id, side, stratum, weight

strata_summary

Tibble with per-stratum counts

info

List with n_strata, n_matched_left, n_matched_right, n_pruned_left, n_pruned_right, method, vars

Details

CEM algorithm:

  1. Coarsen each numeric variable using cut with either user-specified breakpoints or automatic binning (Sturges, FD, or Scott rule)

  2. Categorical variables in grouping are kept as-is

  3. Create strata by concatenating all coarsened values

  4. Drop strata with 0 left or 0 right units

  5. Compute CEM weights: left units get weight 1, right units get weight n_left_in_stratum / n_right_in_stratum so that the total weight of right units in each stratum equals the number of left units

Examples

set.seed(42)
left <- data.frame(
  id = 1:20, age = rnorm(20, 40, 10),
  income = rnorm(20, 50000, 10000)
)
right <- data.frame(
  id = 21:60, age = rnorm(40, 42, 10),
  income = rnorm(40, 52000, 10000)
)
result <- cem_match(left, right, vars = c("age", "income"))
print(result)