Experimental studies in the social and behavioral sciences typically follow a fixed experimental [Page 1347]design where the sample size and composition (e.g., experimental group allocation) is determined prior to conducting the experiment. In contrast, sequential experimental designs treat the sample size as a random variable by allowing sequential interim analyses and decision making based on cumulative data and previous design decisions while maintaining appropriate control over experiment-wise errors in decision making (i.e., Type I [α] and Type II [β] error rates). Also referred to as adaptive or flexible designs, current design decisions in sequential designs are sequentially selected according to previous design points. Sequential designs rely on the principal of stochastic curtailment to stop the experiment if the given data at an interim analysis are ...