AIPW {AIPW} | R Documentation |

## Augmented Inverse Probability Weighting (AIPW)

### Description

An R6Class of AIPW for estimating the average causal effects with users' inputs of exposure, outcome, covariates and related libraries for estimating the efficient influence function.

### Details

An AIPW object is constructed by `new()`

with users' inputs of data and causal structures, then it `fit()`

the data using the
libraries in `Q.SL.library`

and `g.SL.library`

with `k_split`

cross-fitting, and provides results via the `summary()`

method.
After using `fit()`

and/or `summary()`

methods, propensity scores and inverse probability weights by exposure status can be
examined with `plot.p_score()`

and `plot.ip_weights()`

, respectively.

If outcome is missing, analysis assumes missing at random (MAR) by estimating propensity scores of I(A=a, observed=1) with all covariates `W`

.
(`W.Q`

and `W.g`

are disabled.) Missing exposure is not supported.

See examples for illustration.

### Value

`AIPW`

object

### Constructor

`AIPW$new(Y = NULL, A = NULL, W = NULL, W.Q = NULL, W.g = NULL, Q.SL.library = NULL, g.SL.library = NULL, k_split = 10, verbose = TRUE, save.sl.fit = FALSE)`

#### Constructor Arguments

Argument | Type | Details |

`Y` | Integer | A vector of outcome (binary (0, 1) or continuous) |

`A` | Integer | A vector of binary exposure (0 or 1) |

`W` | Data | Covariates for both exposure and outcome models. |

`W.Q` | Data | Covariates for the outcome model (Q). |

`W.g` | Data | Covariates for the exposure model (g). |

`Q.SL.library` | SL.library | Algorithms used for the outcome model (Q). |

`g.SL.library` | SL.library | Algorithms used for the exposure model (g). |

`k_split` | Integer | Number of folds for splitting (Default = 10). |

`verbose` | Logical | Whether to print the result (Default = TRUE) |

`save.sl.fit` | Logical | Whether to save Q.fit and g.fit (Default = FALSE) |

#### Constructor Argument Details

`W`

,`W.Q`

&`W.g`

It can be a vector, matrix or data.frame. If and only if

`W == NULL`

,`W`

would be replaced by`W.Q`

and`W.g`

.`Q.SL.library`

&`g.SL.library`

Machine learning algorithms from SuperLearner libraries

`k_split`

It ranges from 1 to number of observation-1. If k_split=1, no cross-fitting; if k_split>=2, cross-fitting is used (e.g.,

`k_split=10`

, use 9/10 of the data to estimate and the remaining 1/10 leftover to predict).**NOTE: it's recommended to use cross-fitting.**`save.sl.fit`

This option allows users to save the fitted sl object (libs$Q.fit & libs$g.fit) for debug use.

**Warning: Saving the SuperLearner fitted object may cause a substantive storage/memory use.**

### Public Methods

Methods | Details | Link |

`fit()` | Fit the data to the AIPW object | fit.AIPW |

`stratified_fit()` | Fit the data to the AIPW object stratified by `A` | stratified_fit.AIPW |

`summary()` | Summary of the average treatment effects from AIPW | summary.AIPW_base |

`plot.p_score()` | Plot the propensity scores by exposure status | plot.p_score |

`plot.ip_weights()` | Plot the inverse probability weights using truncated propensity scores | plot.ip_weights |

### Public Variables

Variable | Generated by | Return |

`n` | Constructor | Number of observations |

`stratified_fitted` | `stratified_fit()` | Fit the outcome model stratified by exposure status |

`obs_est` | `fit()` & `summary()` | Components calculating average causal effects |

`estimates` | `summary()` | A list of Risk difference, risk ratio, odds ratio |

`result` | `summary()` | A matrix contains RD, ATT, ATC, RR and OR with their SE and 95%CI |

`g.plot` | `plot.p_score()` | A density plot of propensity scores by exposure status |

`ip_weights.plot` | `plot.ip_weights()` | A box plot of inverse probability weights |

`libs` | `fit()` | SuperLearner libraries and their fitted objects |

`sl.fit` | Constructor | A wrapper function for fitting SuperLearner |

`sl.predict` | Constructor | A wrapper function using `sl.fit` to predict |

#### Public Variable Details

`stratified_fit`

An indicator for whether the outcome model is fitted stratified by exposure status in the

`fit()`

method. Only when using`stratified_fit()`

to turn on`stratified_fit = TRUE`

,`summary`

outputs average treatment effects among the treated and the controls.`obs_est`

After using

`fit()`

and`summary()`

methods, this list contains the propensity scores (`p_score`

), counterfactual predictions (`mu`

,`mu1`

&`mu0`

) and efficient influence functions (`aipw_eif1`

&`aipw_eif0`

) for later average treatment effect calculations.`g.plot`

This plot is generated by

`ggplot2::geom_density`

`ip_weights.plot`

This plot uses truncated propensity scores stratified by exposure status (

`ggplot2::geom_boxplot`

)

### References

Zhong Y, Kennedy EH, Bodnar LM, Naimi AI (2021, In Press). AIPW: An R Package for Augmented Inverse Probability Weighted Estimation of Average Causal Effects. *American Journal of Epidemiology*.

Robins JM, Rotnitzky A (1995). Semiparametric efficiency in multivariate regression models with missing data. *Journal of the American Statistical Association*.

Chernozhukov V, Chetverikov V, Demirer M, et al (2018). Double/debiased machine learning for treatment and structural parameters. *The Econometrics Journal*.

Kennedy EH, Sjolander A, Small DS (2015). Semiparametric causal inference in matched cohort studies. *Biometrika*.

### Examples

```
library(SuperLearner)
library(ggplot2)
#create an object
aipw_sl <- AIPW$new(Y=rbinom(100,1,0.5), A=rbinom(100,1,0.5),
W.Q=rbinom(100,1,0.5), W.g=rbinom(100,1,0.5),
Q.SL.library="SL.mean",g.SL.library="SL.mean",
k_split=1,verbose=FALSE)
#fit the object
aipw_sl$fit()
# or use `aipw_sl$stratified_fit()` to estimate ATE and ATT/ATC
#calculate the results
aipw_sl$summary(g.bound = 0.025)
#check the propensity scores by exposure status after truncation
aipw_sl$plot.p_score()
```

*AIPW*version 0.6.3.2 Index]