weightit() allows for the easy generation of balancing weights using a variety of available methods for binary, continuous, and multi-category treatments. Many of these methods exist in other packages, which weightit() calls; these packages must be installed to use the desired method. Also included are print() and summary() methods for examining the output.

## Usage

weightit(formula,
data = NULL,
method = "glm",
estimand = "ATE",
stabilize = FALSE,
focal = NULL,
by = NULL,
s.weights = NULL,
ps = NULL,
moments = NULL,
int = FALSE,
subclass = NULL,
missing = NULL,
verbose = FALSE,
include.obj = FALSE,
...)

# S3 method for weightit
print(x, ...)

## Arguments

formula

a formula with a treatment variable on the left hand side and the covariates to be balanced on the right hand side. See glm() for more details. Interactions and functions of covariates are allowed.

data

an optional data set in the form of a data frame that contains the variables in formula.

method

a string of length 1 containing the name of the method that will be used to estimate weights. See Details below for allowable options. The default is "glm" for propensity score weighting using a generalized linear model to estimate the propensity score.

estimand

the desired estimand. For binary and multi-category treatments, can be "ATE", "ATT", "ATC", and, for some methods, "ATO", "ATM", or "ATOS". The default for both is "ATE". This argument is ignored for continuous treatments. See the individual pages for each method for more information on which estimands are allowed with each method and what literature to read to interpret these estimands.

stabilize

logical; whether or not to stabilize the weights. For the methods that involve estimating propensity scores, this involves multiplying each unit's weight by the proportion of units in their treatment group. Default is FALSE.

focal

when multi-category treatments are used and ATT weights are requested, which group to consider the "treated" or focal group. This group will not be weighted, and the other groups will be weighted to be more like the focal group. If specified, estimand will automatically be set to "ATT".

by

a string containing the name of the variable in data for which weighting is to be done within categories or a one-sided formula with the stratifying variable on the right-hand side. For example, if by = "gender" or by = ~gender, weights will be generated separately within each level of the variable "gender". (The argument used to be called exact, which will still work but with a message.) Only one by variable is allowed; to stratify by multiply variables simultaneously, create a new variable that is a full cross of those variables using interaction().

s.weights

A vector of sampling weights or the name of a variable in data that contains sampling weights. These can also be matching weights if weighting is to be used on matched data. See the individual pages for each method for information on whether sampling weights can be supplied.

ps

A vector of propensity scores or the name of a variable in data containing propensity scores. If not NULL, method is ignored, and the propensity scores will be used to create weights. formula must include the treatment variable in data, but the listed covariates will play no role in the weight estimation. Using ps is similar to calling get_w_from_ps() directly, but produces a full weightit object rather than just producing weights.

moments

numeric; for some methods, the greatest power of each covariate to be balanced. For example, if moments = 3, for each non-categorical covariate, the covariate, its square, and its cube will be balanced. This argument is ignored for other methods; to balance powers of the covariates, appropriate functions must be entered in formula. See the individual pages for each method for information on whether they accept moments.

int

logical; for some methods, whether first-order interactions of the covariates are to be balanced. This argument is ignored for other methods; to balance interactions between the variables, appropriate functions must be entered in formula. See the individual pages for each method for information on whether they accept int.

subclass

numeric; the number of subclasses to use for computing weights using marginal mean weighting with subclasses (MMWS). If NULL, standard inverse probability weights (and their extensions) will be computed; if a number greater than 1, subclasses will be formed and weights will be computed based on subclass membership. Attempting to set a non-NULL value for methods that don't compute a propensity score will result in an error; see each method's help page for information on whether MMWS weights are compatible with the method. See get_w_from_ps() for details and references.

missing

character; how missing data should be handled. The options and defaults depend on the method used. Ignored if no missing data is present. It should be noted that multiple imputation outperforms all available missingness methods available in weightit() and should probably be used instead. Consider the MatchThem package for the use of weightit() with multiply imputed data.

verbose

logical; whether to print additional information output by the fitting function.

include.obj

logical; whether to include in the output any fit objects created in the process of estimating the weights. For example, with method = "glm", the glm objects containing the propensity score model will be included. See the individual pages for each method for information on what object will be included if TRUE.

...

other arguments for functions called by weightit() that control aspects of fitting that are not covered by the above arguments. See Details.

x

a weightit object; the output of a call to weightit().

## Value

A weightit object with the following elements:

weights

The estimated weights, one for each unit.

treat

The values of the treatment variable.

covs

The covariates used in the fitting. Only includes the raw covariates, which may have been altered in the fitting process.

estimand

The estimand requested.

method

The weight estimation method specified.

ps

The estimated or provided propensity scores. Estimated propensity scores are returned for binary treatments and only when method is "glm", "gbm", "cbps", "super", or "bart".

s.weights

The provided sampling weights.

focal

The focal variable if the ATT was requested with a multi-category treatment.

by

A data.frame containing the by variable when specified.

obj

When include.obj = TRUE, the fit object.

info

Additional information about the fitting. See the individual methods pages for what is included.

## Details

The primary purpose of weightit() is as a dispatcher to functions that perform the estimation of balancing weights using the requested method. Below are the methods allowed and links to pages containing more information about them, including additional arguments and outputs (e.g., when include.obj = TRUE), how missing values are treated, which estimands are allowed, and whether sampling weights are allowed.

• "glm" - Propensity score weighting using generalized linear models.

• "gbm" - Propensity score weighting using generalized boosted modeling.

• "cbps" - Covariate Balancing Propensity Score weighting.

• "npcbps" - Non-parametric Covariate Balancing Propensity Score weighting.

• "ebal" - Entropy balancing.

• "optweight" - Optimization-based weighting.

• "super" - Propensity score weighting using SuperLearner.

• "bart" - Propensity score weighting using Bayesian additive regression trees (BART).

• "energy" - Energy balancing.

method can also be supplied as a user-defined function; see method_user for instructions and examples.

When using weightit(), please cite both the WeightIt package (using citation("WeightIt")) and the paper(s) in the references section of the method used.

weightitMSM() for estimating weights with sequential (i.e., longitudinal) treatments for use in estimating marginal structural models (MSMs).

weightit.fit(), which is a lower-level dispatcher function that accepts a matrix of covariates and a vector of treatment statuses rather than a formula and data frame and performs minimal argument checking and processing. It may be useful for speeding up simulation studies for which the correct arguments are known. In general weightit() should be used.

Noah Greifer

## Examples

library("cobalt")
data("lalonde", package = "cobalt")

#Balancing covariates between treatment groups (binary)
(W1 <- weightit(treat ~ age + educ + married +
nodegree + re74, data = lalonde,
method = "glm", estimand = "ATT"))
#> A weightit object
#>  - method: "glm" (propensity score weighting with GLM)
#>  - number of obs.: 614
#>  - sampling weights: none
#>  - treatment: 2-category
#>  - estimand: ATT (focal: 1)
#>  - covariates: age, educ, married, nodegree, re74
summary(W1)
#>                  Summary of weights
#>
#> - Weight ranges:
#>
#>            Min                                  Max
#> treated 1.0000               ||              1.0000
#> control 0.0222 |---------------------------| 2.0438
#>
#> - Units with the 5 most extreme weights by group:
#>
#>              10      8      4      3      1
#>  treated      1      1      1      1      1
#>             411    595    269    409    296
#>  control 1.3303 1.4365 1.5005 1.6369 2.0438
#>
#> - Weight statistics:
#>
#>         Coef of Var   MAD Entropy # Zeros
#> treated       0.000 0.000   -0.00       0
#> control       0.823 0.701    0.33       0
#>
#> - Effective Sample Sizes:
#>
#>            Control Treated
#> Unweighted  429.       185
#> Weighted    255.99     185
bal.tab(W1)
#> Call
#>  weightit(formula = treat ~ age + educ + married + nodegree +
#>     re74, data = lalonde, method = "glm", estimand = "ATT")
#>
#> Balance Measures
#> prop.score Distance   0.0199
#> age         Contin.   0.0459
#> educ        Contin.  -0.0360
#> married      Binary   0.0044
#> nodegree     Binary   0.0080
#> re74        Contin.  -0.0275
#>
#> Effective sample sizes
#>            Control Treated
#Balancing covariates with respect to race (multi-category)
(W2 <- weightit(race ~ age + educ + married +
nodegree + re74, data = lalonde,
method = "ebal", estimand = "ATE"))
#> A weightit object
#>  - method: "ebal" (entropy balancing)
#>  - number of obs.: 614
#>  - sampling weights: none
#>  - treatment: 3-category (black, hispan, white)
#>  - estimand: ATE
#>  - covariates: age, educ, married, nodegree, re74
summary(W2)
#>                  Summary of weights
#>
#> - Weight ranges:
#>
#>           Min                                  Max
#> black  0.5530   |-------------------------| 5.3496
#> hispan 0.1409 |----------------|            3.3322
#> white  0.3979  |-------|                    1.9224
#>
#> - Units with the 5 most extreme weights by group:
#>
#>            226    244    485    181    182
#>   black 2.5215 2.5491 2.8059 3.5551 5.3496
#>            392    564    269    345    371
#>  hispan 2.0467   2.53 2.6322 2.7049 3.3322
#>             68    457    599    589    531
#>   white 1.7106 1.7226 1.7426 1.7743 1.9224
#>
#> - Weight statistics:
#>
#>        Coef of Var   MAD Entropy # Zeros
#> black        0.590 0.413   0.131       0
#> hispan       0.609 0.440   0.163       0
#> white        0.371 0.306   0.068       0
#>
#> - Effective Sample Sizes:
#>
#>             black hispan  white
#> Unweighted 243.    72.   299.
#> Weighted   180.47  52.71 262.93
bal.tab(W2)
#> Call
#>  weightit(formula = race ~ age + educ + married + nodegree + re74,
#>     data = lalonde, method = "ebal", estimand = "ATE")
#>
#> Balance summary across all treatment pairs
#> age      Contin.       0.0001
#> educ     Contin.       0.0000
#> married   Binary       0.0001
#> nodegree  Binary       0.0001
#> re74     Contin.       0.0001
#>
#> Effective sample sizes
#>             black hispan  white
#Balancing covariates with respect to re75 (continuous)
(W3 <- weightit(re75 ~ age + educ + married +
nodegree + re74, data = lalonde,
method = "cbps", over = FALSE))
#> A weightit object
#>  - method: "cbps" (covariate balancing propensity score weighting)
#>  - number of obs.: 614
#>  - sampling weights: none
#>  - treatment: continuous
#>  - covariates: age, educ, married, nodegree, re74
summary(W3)
#>                  Summary of weights
#>
#> - Weight ranges:
#>
#>        Min                                   Max
#> all 0.0153 |---------------------------| 13.1539
#>
#> - Units with the 5 most extreme weights:
#>
#>         484     482     180     481     483
#>  all 9.2281 10.8362 11.0895 11.9898 13.1539
#>
#> - Weight statistics:
#>
#>     Coef of Var   MAD Entropy # Zeros
#> all       1.151 0.449   0.288       0
#>
#> - Effective Sample Sizes:
#>
#>             Total
#> Unweighted 614.
#> Weighted   264.37
bal.tab(W3)
#> Call
#>  weightit(formula = re75 ~ age + educ + married + nodegree + re74,
#>     data = lalonde, method = "cbps", over = FALSE)
#>
#> Balance Measures