Quasirandomization

Quasirandomization is a term used in experimental research to describe the assignment of subjects to treatment conditions using methods that are not strictly random but are designed to approximate randomness. It includes deterministic rules and pseudo-random processes intended to balance groups or simplify logistics while avoiding full randomization. Common forms include alternate allocation, fixed sequences (for example ABAB), allocation by characteristics such as date of enrollment, chart or medical record number, or other quasi-random rules. In many cases, such methods aim to be transparent and easier to implement in field settings, but they may introduce predictability and selection bias if investigators or participants can anticipate the assignment.

In contrast to true randomization, which uses a random mechanism with allocation concealment, quasirandomization can compromise allocation concealment and validity if the assignment becomes predictable. To mitigate bias, researchers may combine quasi-random methods with randomized elements, or use minimization/adaptive allocation that, while not purely random, aims to balance covariates across groups with unpredictable elements.

Applications and debates: Quasirandomization is discussed in clinical trials, education and social experiments, and policy evaluations, particularly where strict randomization is impractical. Analysts may adjust for observed baseline differences and apply intention-to-treat analyses, though residual imbalance can remain.

Relation to quasi-experimental designs: Quasirandomization is distinct from quasi-experimental designs that exploit natural experiments or pre-existing rule-based allocations; it lies closer to experimental methods but with relaxed randomness.