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# coresynth <img src="man/figures/logo.png" align="right" height="140" />

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**coresynth** is a high-performance R package that provides six causal
inference methods for panel data through a unified formula interface.
All core optimizations (QP solving, SVD, Kalman filtering) are
implemented in C++ via RcppArmadillo, so estimation stays fast even on
larger donor pools (see the [Performance](#performance) section for
timings).

## Installation

``` r
# From CRAN
install.packages("coresynth")

# Development version from GitHub
pak::pak("yo5uke/coresynth")
```

## Quick Start

coresynth fits causal panel models through a single formula interface,
`y ~ d | id + time`. This section walks through the classic Synthetic
Control Method (SCM); the same formula works for five other methods (see
[Supported Methods](#supported-methods)).

Your data should be a **balanced panel in long format** (one row per
unit × time), with:

- `y` — the outcome variable
- `d` — the treatment indicator: `0` normally, `1` for a treated unit in
  periods at or after its treatment starts
- `id` — the unit identifier
- `time` — the time identifier

``` r
library(coresynth)

# Generate a balanced panel (10 units, 20 periods, true ATT = 2.0)
set.seed(42)
N <- 10; TT <- 20; T_pre <- 10
f   <- cumsum(rnorm(TT, 0, 0.5))
lam <- rnorm(N, 1, 0.3)
dat <- expand.grid(time = seq_len(TT), id = paste0("u", seq_len(N)))
dat$y <- as.vector(outer(f, lam)) + rnorm(nrow(dat), 0, 0.3)
dat$d <- as.integer(dat$id == "u1" & dat$time > T_pre)   # u1 treated from t=11 on
dat$y[dat$d == 1] <- dat$y[dat$d == 1] + 2.0              # true ATT = 2

fit_scm <- scm_fit(y ~ d | id + time, data = dat, method = "scm")
summary(fit_scm)
#> === coresynth summary ===
#> Method : SCM 
#> Periods : T_pre = 10 | T_post = 10 
#> ATT estimate: 2.271125 
#> Unit weights (non-zero donors):
#>     u2     u4     u6     u7    u10 
#> 0.2475 0.0706 0.1477 0.3277 0.2065
```

The key arguments of `scm_fit()`:

- `formula` — `y ~ d | id + time`, matching the columns above
- `data` — the long-format panel
- `method` — the estimator to use (`"scm"`, `"sdid"`, `"gsc"`, `"mc"`,
  `"tasc"`, or `"si"`); all take the same formula

Many more arguments are available for covariate adjustment, donor
selection, and staggered adoption — see the [Covariates](#covariates)
and [Staggered Adoption](#staggered-adoption) sections below, or
`?scm_fit`.

``` r
# Observed vs. synthetic trend
plot(fit_scm, type = "trend")
```

<img src="man/figures/README-plot-trend-1.png" alt="" width="100%" />

``` r
# Treatment effect over time
plot(fit_scm, type = "gap")
```

<img src="man/figures/README-plot-gap-1.png" alt="" width="100%" />

``` r
# Donor unit weights
plot(fit_scm, type = "weights")
```

<img src="man/figures/README-plot-weights-1.png" alt="" width="100%" />

## Supported Methods

The same formula and data work for all six methods — just change
`method`:

``` r
methods <- c("scm", "sdid", "gsc", "mc", "tasc", "si")
fits    <- lapply(methods, \(m) scm_fit(y ~ d | id + time, data = dat, method = m))
names(fits) <- methods

# Compare ATT estimates (true value = 2.0)
data.frame(
  method   = methods,
  estimate = round(sapply(fits, `[[`, "estimate"), 3)
)
#>      method estimate
#> scm     scm    2.271
#> sdid   sdid    2.150
#> gsc     gsc    2.255
#> mc       mc    2.696
#> tasc   tasc    1.154
#> si       si    2.346
```

| Method | Reference | Treatment | Covariates | Inference |
|----|----|----|:--:|----|
| `scm` | Abadie, Diamond & Hainmueller (2010) | Sharp & Staggered | `pred()` list | `mspe_ratio_pval()`, `conformal_inference()` |
| `sdid` | Arkhangelsky et al. (2021) | Sharp & Staggered | `covariates=` | `sdid_inference()`, `conformal_inference()` |
| `gsc` | Xu (2017) | Sharp & Staggered | `covariates=` time-varying | `gsc_boot()`, `gsc_inference()`, `conformal_inference()` |
| `mc` | Athey et al. (2021) | Sharp & Staggered | — | `conformal_inference()` |
| `tasc` | Rho et al. (2026) | Sharp & Staggered | — | — |
| `si` | Agarwal et al. (2025) | Sharp, Staggered & Multi-arm | — | `si_inference()`, `conformal_inference()` |

`conformal_inference()` (Chernozhukov, Wüthrich & Zhu 2021) provides
permutation-based p-values and confidence intervals for **sharp** fits
across `scm`/`sdid`/`gsc`/`mc`/`si`.

## Staggered Adoption

All six methods support staggered adoption using a cohort-based approach
(Clarke et al. 2023): each adoption cohort is fitted separately and the
cohort ATTs are aggregated with weights proportional to
`N_treated × T_post`.

``` r
# u1: treated from t=11, u2: treated from t=16
dat_s        <- dat
dat_s$d      <- 0L
dat_s$d[dat_s$id == "u1" & dat_s$time > 10] <- 1L
dat_s$d[dat_s$id == "u2" & dat_s$time > 15] <- 1L
dat_s$y[dat_s$d == 1] <- dat_s$y[dat_s$d == 1] + 2.0

# All methods detect staggered timing automatically
fit_sdid <- scm_fit(y ~ d | id + time, data = dat_s, method = "sdid")
fit_gsc  <- scm_fit(y ~ d | id + time, data = dat_s, method = "gsc")
fit_mc   <- scm_fit(y ~ d | id + time, data = dat_s, method = "mc")
fit_si   <- scm_fit(y ~ d | id + time, data = dat_s, method = "si")

# Cohort-level estimates are accessible
fit_sdid$cohort_estimates
#>   cohort estimate weight n_treated T_pre T_post
#> 1     11    1.97  0.667         1    10      9
#> 2     16    2.03  0.333         1    10      4

# control_group = "clean" (default) uses never-treated + future-adopters as donors
# control_group = "never_treated" restricts to never-treated only
fit_sdid_clean <- scm_fit(y ~ d | id + time, data = dat_s, method = "sdid",
                          control_group = "never_treated")
```

## Covariates

### SCM: Predictor Variables via `pred()`

SCM supports covariate-based matching following Abadie et al. (2010)
S.2.3. Use `pred(vars, times, op)` to specify which variables and time
windows to include in the predictor matrix:

``` r
# Assume dat has extra columns: income, unemp
fit_scm_cov <- scm_fit(
  y ~ d | id + time,
  data   = dat,
  method = "scm",
  predictors = list(
    pred(c("income", "unemp"), 1:8),   # average income & unemp over pre-period
    pred("y", 5),                       # outcome at a specific pre-treatment year
    pred("y", 1:4, op = "mean")         # outcome averaged over early pre-period
  )
)
summary(fit_scm_cov)   # shows predictor balance table
```

Each `pred()` call aggregates one or more variables over a time window
using `op = "mean"` (default), `"median"`, or `"sum"`. Multiple `pred()`
calls with different windows can be combined freely in the list.

### GSC: Time-Varying Covariates

GSC supports time-varying covariate adjustment via the full EM algorithm
of Xu (2017). Pass a character vector of column names as `covariates`:

``` r
# Assume dat has a time-varying column: gdp_growth
fit_gsc_cov <- scm_fit(
  y ~ d | id + time,
  data       = dat,
  method     = "gsc",
  r          = 2,
  covariates = "gdp_growth"
)
fit_gsc_cov$beta   # estimated beta coefficient(s)
```

The EM loop alternates between:

- **E-step**: SVD of $\tilde{Y}_{it} = Y_{it} - x_{it}'\hat\beta$ to
  update factors $\hat{F}$, $\hat\Lambda$
- **M-step**: Ridge OLS of residuals on $x_{it}$ to update $\hat\beta$

Treated unit loadings are estimated from covariate-demeaned
pre-treatment data per Xu (2017) Step 2. When `covariates = NULL`
(default), the plain 3-step SVD estimator ($\hat\beta = 0$) is used.

## Inference

### SCM: In-Space Placebo Test

`mspe_ratio_pval()` runs the in-space placebo study of Abadie, Diamond &
Hainmueller (2010): the intervention is reassigned to each donor unit
and the treated unit’s post/pre-treatment MSPE ratio is compared against
the placebo distribution. The result can be plotted directly.

``` r
scm_p <- mspe_ratio_pval(fits$scm)
scm_p$p_value
#> [1] 0.1

# Treated gap vs. placebo gaps in the donor pool (ADH 2010, Figures 4-7)
plot(scm_p, type = "gaps")
```

<img src="man/figures/README-placebo-gaps-1.png" alt="" width="100%" />

Placebo units with a poor pre-treatment fit can be pruned with
`mspe_prune` (e.g. `plot(scm_p, type = "gaps", mspe_prune = 5)` excludes
units whose pre-treatment MSPE exceeds 5 times the treated unit’s).

``` r
# Post/pre-treatment MSPE ratio of every unit (ADH 2010, Figure 8)
plot(scm_p, type = "ratios")
```

<img src="man/figures/README-placebo-ratios-1.png" alt="" width="100%" />

### Other Inference Methods

``` r
# SDID: four inference methods — placebo, bootstrap, jackknife, jackknife_global
sdid_inf <- sdid_inference(fits$sdid, method = "placebo")
tidy(sdid_inf)   # broom-style one-row data.frame

sdid_boot <- sdid_inference(fits$sdid, method = "bootstrap", n_boot = 200, seed = 1)
tidy(sdid_boot)

# GSC: parametric bootstrap under H0 (sharp only)
gsc_ci <- gsc_boot(fits$gsc, B = 200, alpha = 0.05)
cat("95% CI: [", gsc_ci$ci_lower, ",", gsc_ci$ci_upper, "]\n")

# GSC / SI: non-parametric inference (sharp + staggered)
gsc_inf <- gsc_inference(fits$gsc, method = "jackknife")
si_inf  <- si_inference(fits$si,  method = "bootstrap", n_boot = 200, seed = 1)
tidy(gsc_inf)
tidy(si_inf)

# Conformal inference (Chernozhukov, Wüthrich & Zhu 2021) — sharp fits for
# scm / sdid / gsc / mc / si. Re-estimates the counterfactual under the null
# and inverts a moving-block permutation test for a p-value and CI.
conf <- conformal_inference(fits$scm, tau0 = 0, level = 0.95)
tidy(conf)
```

## tidyverse / broom Integration

``` r
library(broom)

# Extract weights as a data frame
tidy(fits$scm)

# Summary row
glance(fits$scm)

# JSON export (for reproducibility and AI workflows)
export_json(fits$scm, file = "scm_result.json")
```

## Performance

Estimation stays fast even as the donor pool grows. SCM fit times
(Windows 11 / R 4.6.0 / GCC 14.2.0):

| N_co | T_pre | coresynth |
|-----:|------:|----------:|
|   16 |    10 |    105 ms |
|   20 |    20 |     72 ms |
|   50 |    30 |    916 ms |
|  100 |    40 |  6,300 ms |

A full SCM fit on a 100-donor pool completes in a few seconds.

## References

- Abadie, A., Diamond, A., & Hainmueller, J. (2010). Synthetic control
  methods for comparative case studies. *JASA*, 105(490), 493–505.
- Agarwal, A., Shah, D., & Shen, D. (2025). Synthetic interventions:
  extending synthetic controls to multiple treatments. *Operations
  Research*. <doi:10.1287/opre.2025.1590>
- Arkhangelsky, D., Athey, S., Hirshberg, D. A., Imbens, G. W., &
  Wager, S. (2021). Synthetic difference-in-differences. *AER*, 111(12),
  4088–4118.
- Athey, S., Bayati, M., Doudchenko, N., Imbens, G., & Khosravi, K.
  (2021). Matrix completion methods for causal panel data models.
  *JASA*, 116(536), 1716–1730.
- Chernozhukov, V., Wüthrich, K., & Zhu, Y. (2021). An exact and robust
  conformal inference method for counterfactual and synthetic controls.
  *JASA*, 116(536), 1849–1864.
- Rho, H., et al. (2026). Time-aware synthetic control. *Working paper*.
- Xu, Y. (2017). Generalized synthetic control method. *Political
  Analysis*, 25(1), 57–76.
