Horvitz-Thompson estimator for two-armed trials

Horvitz-Thompson estimators that are unbiased for designs in which the randomization scheme is known

Usage

horvitz_thompson(
  formula,
  data,
  blocks,
  clusters,
  simple = NULL,
  condition_prs,
  condition_pr_mat = NULL,
  ra_declaration = NULL,
  subset,
  condition1 = NULL,
  condition2 = NULL,
  se_type = c("youngs", "constant", "none"),
  ci = TRUE,
  alpha = 0.05,
  return_condition_pr_mat = FALSE
)

Arguments

formula: an object of class formula, as in lm, such as Y ~ Z with only one variable on the right-hand side, the treatment.
data: A data.frame.
blocks: An optional bare (unquoted) name of the block variable. Use for blocked designs only. See details.
clusters: An optional bare (unquoted) name of the variable that corresponds to the clusters in the data; used for cluster randomized designs. For blocked designs, clusters must be within blocks.
simple: logical, optional. Whether the randomization is simple (TRUE) or complete (FALSE). This is ignored if blocks are specified, as all blocked designs use complete randomization, or either ra_declaration or condition_pr_mat are passed. Otherwise, it defaults to TRUE.
condition_prs: An optional bare (unquoted) name of the variable with the condition 2 (treatment) probabilities. See details. May also use a single number for the condition 2 probability if it is constant.
condition_pr_mat: An optional 2n * 2n matrix of marginal and joint probabilities of all units in condition1 and condition2. See details.
ra_declaration: An object of class "ra_declaration", from the declare_ra function in the randomizr package. This is the third way that one can specify a design for this estimator. Cannot be used along with any of condition_prs, blocks, clusters, or condition_pr_mat. See details.
subset: An optional bare (unquoted) expression specifying a subset of observations to be used.
condition1: value in the treatment vector of the condition to be the control. Effects are estimated with condition1 as the control and condition2 as the treatment. If unspecified, condition1 is the "first" condition and condition2 is the "second" according to levels if the treatment is a factor or according to a sortif it is a numeric or character variable (i.e if unspecified and the treatment is 0s and 1s, condition1 will by default be 0 and condition2 will be 1). See the examples for more.
condition2: value in the treatment vector of the condition to be the treatment. See condition1.
se_type: can be one of c("youngs", "constant", "none") and corresponds the estimator of the standard errors. Default estimator uses Young's inequality (and is conservative) while the other uses a constant treatment effects assumption and only works for simple randomized designs at the moment. If "none" then standard errors will not be computed which may speed up run time if only the point estimate is required.
ci: logical. Whether to compute and return p-values and confidence intervals, TRUE by default.
alpha: The significance level, 0.05 by default.
return_condition_pr_mat: logical. Whether to return the condition probability matrix. Returns NULL if the design is simple randomization, FALSE by default.

Value

Returns an object of class "horvitz_thompson".

The post-estimation commands functions summary and tidy

return results in a data.frame. To get useful data out of the return, you can use these data frames, you can use the resulting list directly, or you can use the generic accessor functions coef and confint.

An object of class "horvitz_thompson" is a list containing at least the following components:

coefficients: the estimated difference in totals
std.error: the estimated standard error
statistic: the z-statistic
df: the estimated degrees of freedom
p.value: the p-value from a two-sided z-test using coefficients and std.error
conf.low: the lower bound of the 1 - alpha percent confidence interval
conf.high: the upper bound of the 1 - alpha percent confidence interval
term: a character vector of coefficient names
alpha: the significance level specified by the user
nobs: the number of observations used
outcome: the name of the outcome variable
condition_pr_mat: the condition probability matrix if return_condition_pr_mat is TRUE

Details

This function implements the Horvitz-Thompson estimator for treatment effects for two-armed trials. This estimator is useful for estimating unbiased treatment effects given any randomization scheme as long as the randomization scheme is known.

In short, the Horvitz-Thompson estimator essentially reweights each unit by the probability of it being in its observed condition. Pivotal to the estimation of treatment effects using this estimator are the marginal condition probabilities (i.e., the probability that any one unit is in a particular treatment condition). Pivotal to the estimating the variance variance whenever the design is more complicated than simple randomization, are the joint condition probabilities (i.e., the probabilities that any two units have a particular set of treatment conditions, either the same or different). The estimator we provide here considers the case with two treatment conditions.

Users interested in more details can see the mathematical notes for more information and references, or see the references below.

There are three distinct ways that users can specify the design to the function. The preferred way is to use the declare_ra function in the randomizr package. This function takes several arguments, including blocks, clusters, treatment probabilities, whether randomization is simple or not, and more. Passing the outcome of that function, an object of class "ra_declaration" to the ra_declaration argument in this function, will lead to a call of the declaration_to_condition_pr_mat function which generates the condition probability matrix needed to estimate treatment effects and standard errors. We provide many examples below of how this could be done.

The second way is to pass the names of vectors in your data to condition_prs, blocks, and clusters. You can further specify whether the randomization was simple or complete using the simple argument. Note that if blocks are specified the randomization is always treated as complete. From these vectors, the function learns how to build the condition probability matrix that is used in estimation.

In the case where condition_prs is specified, this function assumes those probabilities are the marginal probability that each unit is in condition2 and then uses the other arguments (blocks, clusters, simple) to learn the rest of the design. If users do not pass condition_prs, this function learns the probability of being in condition2 from the data. That is, none of these arguments are specified, we assume that there was a simple randomization where the probability of each unit being in condition2 was the average of all units in condition2. Similarly, we learn the block-level probability of treatment within blocks by looking at the mean number of units in condition2 if condition_prs is not specified.

The third way is to pass a condition_pr_mat directly. One can see more about this object in the documentation for declaration_to_condition_pr_mat and permutations_to_condition_pr_mat. Essentially, this 2n * 2n matrix allows users to specify marginal and joint marginal probabilities of units being in conditions 1 and 2 of arbitrary complexity. Users should only use this option if they are certain they know what they are doing.

References

Aronow, Peter M, and Joel A Middleton. 2013. "A Class of Unbiased Estimators of the Average Treatment Effect in Randomized Experiments." Journal of Causal Inference 1 (1): 135-54. doi:10.1515/jci-2012-0009 .

Aronow, Peter M, and Cyrus Samii. 2017. "Estimating Average Causal Effects Under Interference Between Units." Annals of Applied Statistics, forthcoming. https://arxiv.org/abs/1305.6156v3.

Middleton, Joel A, and Peter M Aronow. 2015. "Unbiased Estimation of the Average Treatment Effect in Cluster-Randomized Experiments." Statistics, Politics and Policy 6 (1-2): 39-75. doi:10.1515/spp-2013-0002 .

Examples