Design-based variance estimation of the Hájek effect estimator in stratified and clustered experiments

Randomized controlled trials (RCTs) are used to evaluate treatment effects. When individuals are grouped together, clustered RCTs are conducted. Stratification is recommended to reduce imbalance of baseline covariates between treatment and control. In practice, this can lead to comparisons between clusters of very different sizes. As a result, direct adjustment estimators that average differences of means within the strata may be inconsistent. We study differences of inverse probability weighted means of a treatment and a control group -- H\'ajek effect estimators -- under two common forms of stratification: small strata that increase in number; or larger strata with growing numbers of clusters in each. Under either scenario, mild conditions give consistency and asymptotic Normality. We propose a variance estimator applicable to designs with any number of strata and strata of any size. We describe a special use of the variance estimator that improves small sample performance of Wald-type confidence intervals. The H\'ajek effect estimator lends itself to covariance adjustment, and our variance estimator remains applicable. Simulations and real-world applications in children's nutrition and education confirm favorable operating characteristics, demonstrating advantages of the H\'ajek effect estimator beyond its simplicity and ease of use.