Azeem M. Shaikh: all content

Showing 1 – 16 of 16 results

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Inference in Experiments with Matched Pairs

Working Paper

This paper studies inference for the average treatment e ect in randomized controlled trials where treatment status is determined according to a \matched pairs" design.

25 April 2019

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Inference under covariate-adaptive randomization with multiple treatments

Working Paper

This paper studies inference in randomized controlled trials with covariate-adaptive randomization when there are multiple treatments. More speci cally, we study in this setting inference about the average effect of one or more treatments relative to other treatments or a control.

22 January 2019

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Inference under covariate-adaptive randomization with multiple treatments

Working Paper

This paper studies inference in randomized controlled trials with covariate-adaptive randomization when there are multiple treatments. More specifically, we study in this setting inference about the average effect of one or more treatments relative to other treatments or a control. As in Bugni et al. (2017), covariate-adaptive randomization refers to randomization schemes that first stratify according to baseline covariates and then assign treatment status so as to achieve "balance" within each stratum. In contrast to Bugni et al. (2017), however, we allow for the proportion of units being assigned to each of the treatments to vary across strata.

2 August 2017

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Inference under covariate-adaptive randomization

Working Paper

This paper studies inference for the average treatment effect in randomized controlled trials with covariate-adaptive randomization. Here, by covariate-adaptive randomization, we mean randomization schemes that first stratify according to baseline covariates and then assign treatment status so as to achieve "balance" within each stratum.

24 May 2017

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Inference under Covariate-Adaptive Randomization

Working Paper

This paper studies inference for the average treatment eff ect in randomized controlled trials with covariate-adaptive randomization. Here, by covariate-adaptive randomization, we mean randomization schemes that first stratify according to baseline covariates and then assign treatment status so as to achieve "balance" within each stratum. Such schemes include, for example, Efron's biased-coin design and strati ed block randomization. When testing the null hypothesis that the average treatment eff ect equals a pre-speci fied value in such settings, we fi rst show that the usual two-sample t-test is conservative in the sense that it has limiting rejection probability under the null hypothesis no greater than and typically strictly less than the nominal level. In a simulation study, we fi nd that the rejection probability may in fact be dramatically less than the nominal level. We show further that these same conclusions remain true for a naïve permutation test, but that a modi fied version of the permutation test yields a test that is non-conservative in the sense that its limiting rejection probability under the null hypothesis equals the nominal level for a wide variety of randomization schemes. The modi fied version of the permutation test has the additional advantage that it has rejection probability exactly equal to the nominal level for some distributions satisfying the null hypothesis and some randomization schemes. Finally, we show that the usual t-test (on the coefficient on treatment assignment) in a linear regression of outcomes on treatment assignment and indicators for each of the strata yields a non-conservative test as well under even weaker assumptions on the randomization scheme. In a simulation study, we fi nd that the non-conservative tests have substantially greater power than the usual two-sample t-test.

10 May 2016