Screening applies the eligibility criteria for a clinical trial before treatment allocation. It identifies the population to which the comparison pertains and excludes individuals for whom the intervention or measurements are unsuitable. Recording potentially eligible individuals also helps describe recruitment and assess external validity. It should precede randomization so eligibility is not decided using knowledge of the allocated treatment.
Obtain informed consent so that participation in the clinical trial is voluntary and based on an understanding of its procedures, potential benefits and risks, alternatives and withdrawal arrangements. Consent before allocation prevents willingness to participate from being selected according to the known treatment assignment.
Measure baseline covariates before the intervention can change them. These measurements describe the recruited population, permit assessment of covariate balance, and provide prognostic predictors that can improve statistical efficiency. In particular, including prognostic baseline covariates in a linear regression reduces unexplained outcome variation without conditioning on a treatment consequence.
Randomization makes assignment independent of the participants' pre-existing potential outcomes, giving an unbiased comparison of allocation groups under the usual trial assumptions. It therefore protects against baseline confounding, including from unmeasured factors, and supplies an assignment distribution for inference. Allocation concealment protects this step from recruitment decisions based on the upcoming assignment. It does not guarantee exact balance in every realized sample.
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