Quick Start

What couplr Does

couplr creates matched pairs from two groups of observations. Given a “left” group (e.g., treatment) and a “right” group (e.g., control), it finds the best one-to-one pairing based on similarity across variables you specify.

Common use cases:

Matching treated patients to similar controls in observational studies
Pairing survey respondents for comparison
Creating balanced samples for causal inference

Documentation Roadmap

Vignette	Focus	Audience
Quick Start (this)	Basic matching with `match_couples()`	Everyone
Matching Workflows	Full pipeline: preprocessing, blocking, diagnostics	Researchers
Algorithms	Mathematical foundations, solver selection	Technical users
Comparison	vs MatchIt, optmatch, designmatch	Package evaluators

Start here, then proceed to whichever vignette matches your use case.

Your First Match

The simplest workflow uses match_couples():

library(couplr)
library(dplyr)

# Create example data: treatment and control groups
set.seed(123)
treatment <- tibble(
  id = 1:50,
  age = rnorm(50, mean = 45, sd = 10),
  income = rnorm(50, mean = 55000, sd = 12000)
)

control <- tibble(
  id = 1:80,
  age = rnorm(80, mean = 50, sd = 12),
  income = rnorm(80, mean = 48000, sd = 15000)
)

# Match on age and income
result <- match_couples(
  left = treatment,
  right = control,
  vars = c("age", "income"),
  auto_scale = TRUE
)
#> Auto-selected scaling method: standardize

# View matched pairs
head(result$pairs)
#> # A tibble: 6 × 5
#>   left_id right_id distance .age_diff .income_diff
#>   <chr>   <chr>       <dbl>     <dbl>        <dbl>
#> 1 1       46          0.779    -4.23        9144. 
#> 2 2       11          0.257    -0.398       3441. 
#> 3 3       52          0.594     1.36        7840. 
#> 4 4       66          0.809    -7.87       -5112. 
#> 5 5       65          0.530     1.30        6977. 
#> 6 6       36          0.130    -1.43          62.2

What happened:

couplr calculated how similar each treatment unit is to each control unit
It found the optimal one-to-one pairing that minimizes total distance
Each treatment unit gets matched to exactly one control unit

Understanding the Output

# Quick overview with summary()
summary(result)
#> Matching Result Summary
#> =======================
#> 
#> Method: lap
#> Pairs matched: 50 
#> Unmatched: 0 left, 30 right
#> 
#> Distance Statistics:
#>   Total: 19.6423 
#>   Mean: 0.3928 
#>   Min: 0.0615 
#>   Q1: 0.1845 
#>   Median: 0.2967 
#>   Q3: 0.5039 
#>   Max: 1.2184 
#>   SD: 0.2863

# Or access specific info
result$info$n_matched
#> [1] 50

The result$pairs table contains:

left_id: Row number from the treatment group
right_id: Row number from the control group
distance: How different the matched units are (lower = more similar)

Why Scaling Matters

Without scaling, variables with larger values dominate the matching. Income (measured in thousands) would overwhelm age (measured in decades):

# BAD: Without scaling, income dominates
result_unscaled <- match_couples(
  treatment, control,
  vars = c("age", "income"),
  auto_scale = FALSE
)

# GOOD: With scaling, both variables contribute equally
result_scaled <- match_couples(
  treatment, control,
  vars = c("age", "income"),
  auto_scale = TRUE
)
#> Auto-selected scaling method: standardize

# Compare mean distances
cat("Unscaled mean distance:", round(mean(result_unscaled$pairs$distance), 1), "\n")
#> Unscaled mean distance: 2769.1
cat("Scaled mean distance:", round(mean(result_scaled$pairs$distance), 3), "\n")
#> Scaled mean distance: 0.393

Rule of thumb: Always use auto_scale = TRUE unless you have a specific reason not to.

Checking Match Quality

After matching, verify that treatment and control groups are now balanced:

# Get the matched observations
matched_treatment <- treatment[result$pairs$left_id, ]
matched_control <- control[result$pairs$right_id, ]

# Compare means before and after matching
cat("BEFORE matching:\n")
#> BEFORE matching:
cat("  Age difference:", round(mean(treatment$age) - mean(control$age), 1), "years\n")
#>   Age difference: -3.4 years
cat("  Income difference: $", round(mean(treatment$income) - mean(control$income), 0), "\n\n")
#>   Income difference: $ 9266

cat("AFTER matching:\n")
#> AFTER matching:
cat("  Age difference:", round(mean(matched_treatment$age) - mean(matched_control$age), 1), "years\n")
#>   Age difference: -1.3 years
cat("  Income difference: $", round(mean(matched_treatment$income) - mean(matched_control$income), 0), "\n")
#>   Income difference: $ 2667

For formal balance assessment, use balance_diagnostics() (covered in Matching Workflows).

Visualizing Match Quality

Use plot() to see the distribution of match distances:

plot(result)

The histogram shows how similar matched pairs are. A distribution concentrated near zero indicates good matches.

Large Datasets: Use Greedy Matching

For datasets larger than a few thousand observations, optimal matching becomes slow. Use greedy_couples() instead; it’s 10-100x faster with nearly identical results:

# Create larger datasets
set.seed(456)
large_treatment <- tibble(
  id = 1:2000,
  age = rnorm(2000, 45, 10),
  income = rnorm(2000, 55000, 12000)
)

large_control <- tibble(
  id = 1:3000,
  age = rnorm(3000, 50, 12),
  income = rnorm(3000, 48000, 15000)
)

# Fast greedy matching
result_greedy <- greedy_couples(
  large_treatment, large_control,
  vars = c("age", "income"),
  auto_scale = TRUE,
  strategy = "row_best"  # fastest strategy
)
#> Auto-selected scaling method: standardize

cat("Matched", result_greedy$info$n_matched, "pairs\n")
#> Matched 2000 pairs
cat("Mean distance:", round(mean(result_greedy$pairs$distance), 3), "\n")
#> Mean distance: 0.201

When to use which:

Dataset size	Recommended function
< 1,000 per group	`match_couples()`
1,000 - 5,000	Either works; greedy is faster
> 5,000	`greedy_couples()`

Setting a Maximum Distance (Caliper)

Sometimes you want to reject poor matches rather than force bad pairings. Use max_distance to set a caliper:

# Allow any match
result_loose <- match_couples(
  treatment, control,
  vars = c("age", "income"),
  auto_scale = TRUE
)
#> Auto-selected scaling method: standardize

# Only allow close matches
result_strict <- match_couples(
  treatment, control,
  vars = c("age", "income"),
  auto_scale = TRUE,
  max_distance = 0.5  # reject pairs more different than this
)
#> Auto-selected scaling method: standardize

cat("Without caliper:", result_loose$info$n_matched, "pairs\n")
#> Without caliper: 50 pairs
cat("With caliper:", result_strict$info$n_matched, "pairs\n")
#> With caliper: 40 pairs

Stricter calipers mean fewer but better matches.

Matching Within Groups (Blocking)

When you have natural groups in your data (e.g., hospitals, regions, study sites), you can match within each group separately. This ensures exact balance on the grouping variable.

First, create blocks with matchmaker(), then pass the result to match_couples():

# Data from multiple hospital sites
set.seed(321)
treated <- tibble(
  id = 1:60,
  site = rep(c("Hospital A", "Hospital B", "Hospital C"), each = 20),
  age = rnorm(60, 55, 10),
  severity = rnorm(60, 5, 2)
)

controls <- tibble(
  id = 1:90,
  site = rep(c("Hospital A", "Hospital B", "Hospital C"), each = 30),
  age = rnorm(90, 52, 12),
  severity = rnorm(90, 4.5, 2.5)
)

# Step 1: Create blocks by hospital site
blocks <- matchmaker(
  left = treated,
  right = controls,
  block_type = "group",
  block_by = "site"
)

# Step 2: Match within each block
result_blocked <- match_couples(
  left = blocks$left,
  right = blocks$right,
  vars = c("age", "severity"),
  block_id = "block_id",
  auto_scale = TRUE
)
#> Auto-selected scaling method: standardize

# Verify: matches stay within their block
result_blocked$pairs |> count(block_id)
#> # A tibble: 3 × 2
#>   block_id       n
#>   <chr>      <int>
#> 1 Hospital A    20
#> 2 Hospital B    20
#> 3 Hospital C    20

Blocking guarantees that Hospital A patients are only matched to Hospital A controls, etc.

Complete Example

Here’s a realistic workflow from start to finish:

# 1. Prepare your data
set.seed(789)
patients_treated <- tibble(
  patient_id = paste0("T", 1:100),
  age = rnorm(100, 62, 8),
  bmi = rnorm(100, 28, 4),
  smoker = sample(0:1, 100, replace = TRUE, prob = c(0.6, 0.4))
)

patients_control <- tibble(
  patient_id = paste0("C", 1:200),
  age = rnorm(200, 58, 10),
  bmi = rnorm(200, 26, 5),
  smoker = sample(0:1, 200, replace = TRUE, prob = c(0.7, 0.3))
)

# 2. Match on clinical variables
matched <- match_couples(
  left = patients_treated,
  right = patients_control,
  vars = c("age", "bmi", "smoker"),
  auto_scale = TRUE
)
#> Auto-selected scaling method: standardize

# 3. Check how many matched
cat("Treated patients:", nrow(patients_treated), "\n")
#> Treated patients: 100
cat("Successfully matched:", matched$info$n_matched, "\n")
#> Successfully matched: 100
cat("Match rate:", round(100 * matched$info$n_matched / nrow(patients_treated), 1), "%\n")
#> Match rate: 100 %

# 4. Extract matched samples for analysis
treated_matched <- patients_treated[matched$pairs$left_id, ]
control_matched <- patients_control[matched$pairs$right_id, ]

# 5. Verify balance
cat("\nBalance check (difference in means):\n")
#> 
#> Balance check (difference in means):
cat("  Age:", round(mean(treated_matched$age) - mean(control_matched$age), 2), "\n")
#>   Age: NA
cat("  BMI:", round(mean(treated_matched$bmi) - mean(control_matched$bmi), 2), "\n")
#>   BMI: NA
cat("  Smoker %:", round(100*(mean(treated_matched$smoker) - mean(control_matched$smoker)), 1), "\n")
#>   Smoker %: NA

Next Steps

You now know the basics of matching with couplr. Here’s where to go next:

For production research workflows:

Matching Workflows covers preprocessing, blocking, formal balance diagnostics, and publication-ready output

For understanding algorithm choices:

Algorithms explains when different solvers are faster or more appropriate

For comparing with other packages:

Comparison shows how couplr differs from MatchIt, optmatch, and designmatch

Additional: Direct Assignment Problem Solving

If you need to solve assignment problems directly (not matching workflows), couplr also provides lower-level functions.

lap_solve(): Matrix-Based Assignment

Given a cost matrix where entry (i,j) is the cost of assigning row i to column j:

# Cost matrix: 3 workers × 3 tasks
cost <- matrix(c(
  4, 2, 5,
  3, 3, 6,
  7, 5, 4
), nrow = 3, byrow = TRUE)

result <- lap_solve(cost)
print(result)
#> Assignment Result
#> =================
#> 
#> # A tibble: 3 × 3
#>   source target  cost
#>    <int>  <int> <dbl>
#> 1      1      2     2
#> 2      2      1     3
#> 3      3      3     4
#> 
#> Total cost: 9 
#> Method: bruteforce

Row 1 is assigned to column 2 (cost 2), row 2 to column 1 (cost 3), row 3 to column 3 (cost 4). Total cost: 9.

Forbidden Assignments

Use NA or Inf for impossible assignments:

cost_forbidden <- matrix(c(
  4, 2, NA,   # Row 1 cannot go to column 3
  Inf, 3, 6,  # Row 2 cannot go to column 1
  7, 5, 4
), nrow = 3, byrow = TRUE)

lap_solve(cost_forbidden)
#> Assignment Result
#> =================
#> 
#> # A tibble: 3 × 3
#>   source target  cost
#>    <int>  <int> <dbl>
#> 1      1      1     4
#> 2      2      2     3
#> 3      3      3     4
#> 
#> Total cost: 11 
#> Method: bruteforce

Maximization

For preference or profit maximization:

preferences <- matrix(c(
  8, 5, 3,
  4, 7, 6,
  2, 4, 9
), nrow = 3, byrow = TRUE)

lap_solve(preferences, maximize = TRUE)
#> Assignment Result
#> =================
#> 
#> # A tibble: 3 × 3
#>   source target  cost
#>    <int>  <int> <dbl>
#> 1      1      1     8
#> 2      2      2     7
#> 3      3      3     9
#> 
#> Total cost: 24 
#> Method: bruteforce

Grouped Data

Solve multiple assignment problems at once using grouped data frames:

# Weekly nurse-shift scheduling: solve each day separately
schedule <- tibble(
  day = rep(c("Mon", "Tue", "Wed"), each = 9),
  nurse = rep(rep(1:3, each = 3), 3),
  shift = rep(1:3, 9),
  cost = c(4,2,5, 3,3,6, 7,5,4,   # Monday costs
           5,3,4, 2,4,5, 6,4,3,   # Tuesday costs
           3,4,5, 4,2,6, 5,5,4)   # Wednesday costs
)

# Solve all three days at once
schedule |>
  group_by(day) |>
  lap_solve(nurse, shift, cost)
#> # A tibble: 9 × 4
#>   day   source target  cost
#>   <chr>  <int>  <int> <dbl>
#> 1 Mon        1      2     2
#> 2 Mon        2      1     3
#> 3 Mon        3      3     4
#> 4 Tue        1      2     3
#> 5 Tue        2      1     2
#> 6 Tue        3      3     3
#> 7 Wed        1      1     3
#> 8 Wed        2      2     2
#> 9 Wed        3      3     4

This solves each day’s assignment problem independently and returns all results in one tidy table.

K-Best Solutions

Find multiple near-optimal solutions:

cost <- matrix(c(1, 2, 3, 4, 3, 2, 5, 4, 1), nrow = 3, byrow = TRUE)

kbest <- lap_solve_kbest(cost, k = 3)
print(kbest)
#> K-Best Assignment Results
#> =========================
#> 
#> Number of solutions: 3 
#> 
#> Solution costs:
#>   Rank 1: 5.0000
#>   Rank 2: 7.0000
#>   Rank 3: 7.0000
#> 
#> Assignments:
#> # A tibble: 9 × 6
#>    rank solution_id source target  cost total_cost
#>   <int>       <int>  <int>  <int> <dbl>      <dbl>
#> 1     1           1      1      1     1          5
#> 2     1           1      2      2     3          5
#> 3     1           1      3      3     1          5
#> 4     2           2      1      2     2          7
#> 5     2           2      2      1     4          7
#> 6     2           2      3      3     1          7
#> 7     3           3      1      1     1          7
#> 8     3           3      2      3     2          7
#> 9     3           3      3      2     4          7

Gilles Colling

2026-01-20