Randomization in single-case design experiments: Addressing threats to internal validity

Area of Science:

• Psychology
• Research Methodology
• Behavioral Science

Background:

• Single-case experimental designs (SCEDs) are crucial for evaluating interventions.
• Traditional SCEDs rely heavily on replication to ensure internal validity.
• Threats to internal validity can persist even with replication in SCEDs.

Purpose of the Study:

• To review and detail various randomization strategies applicable to SCED methodology.
• To explicate how specific randomization techniques control threats to internal validity.
• To demonstrate the utility of randomization in both conventional and nonconventional SCEDs.

Main Methods:

• Review of existing literature on randomization in experimental designs.
• Categorization of randomization strategies: phase-order, between-intervention, within-intervention, and start-point randomization.
• Application of these strategies to various SCED variations (e.g., ABAB, alternating treatment, multiple baseline) and non-replication designs.

Main Results:

• Randomization offers explicit control over internal validity threats not fully addressed by replication.
• Specific randomization methods are detailed for diverse SCED structures.
• Randomization enhances scientific credibility, statistical conclusion validity, and likelihood of inclusion in literature syntheses.

Conclusions:

• Randomization is a vital tool for strengthening the internal validity of SCEDs.
• Implementing randomization increases the rigor and statistical power of SCED research.
• Recommendations are made for establishing randomization standards in SCED methodology.