The multiple baseline design involves an effect replication option across participants, settings, or behaviors. Multiple AB data series are compared and introduction of the intervention is staggered across time. In this design, more valid causal inferences are possible by staggering the intervention across one of the aforementioned units (i.e., sequential introduction of the intervention across time). The minimum number of phase repetitions needed to meet the standard advanced by Horner et al. (2005) is three, but four or more is recognized as more desirable (and statistically advantageous in cases in which, for example, the researcher is applying a randomization statistical test). Adding phase repetitions increases the power of the statistical test, similar to adding participants in a traditional group design (Kratochwill & Levin, in press). The number and timing of the repetitions can vary, depending on the outcomes of the intervention. For example, if change in the dependent variable is slow to occur, more time might be needed to demonstrate experimental control. Such a circumstance might also reduce the number of phase repetitions that can be scheduled due to cost and logistical factors. Among the characteristics of this design, effect replication across series is regarded as the characteristic with the greatest potential for enhancing internal and statistical-conclusion validity (see, for example, Levin, 1992). Well-structured SCD research that embraces phase repetition and effect replication can rule out major threats to internal validity. The possible threats to internal validity in single-case research include the following (see also Shadish et al., 2002, p. 55): 1.Ambiguous Temporal Precedence:Lack of clarity about which variable occurred first may yield confusion about which variable is the cause and which is the effect.
8 Embedded in the SCD Standardsis a criterion that the independent variable is actively manipulated by the researcher, with measurement of the dependent variable occurring after that manipulation. This sequencing ensures the presumed cause precedes the presumed effect. A SCD cannot meet Standardsunless there is active manipulation of the independent variable.3Replication of this manipulation-measurement sequence in the experiment further contributes to an argument of unidirectional causation (Shadish et al., 2002). Effect replication, as specified in the Standards, can occur either through within-case replication or multiple-case replication in a single experiment, or by conducting two or more experiments with the same or highly similar intervention conditions included. The Standardsspecify that the study must show a minimum of three demonstrations of the effect through the use of the same design and procedures. Overall, studies that can meet standards are designed to mitigate the threat of ambiguous temporal precedence.