As a general rule, there should be approximately the same length of time and number of measurements in each phase of a single-subject design. There are occasions when the rule of equal phases conflicts with the need to maintain baseline or treatment conditions until a stable pattern of measurements has emerged. It may be necessary to maintain a treatment longer than anticipated for ethical reasons. One way to overcome this problem is to do sufficient pilot testing beforehand to investigate possible treatment parameters. The most satisfactory method for increasing the external validity of single-subject experiments is to conduct replication studies.
Replication can involve variation in investigators, settings, and subjects.
Essential to replication is a careful description of baseline and treatment conditions, subject characteristics, and measurement procedures. Researchers who work with single-subject designs are equally concerned with problems of internal and external validity as are researchers who use group designs. Typically involves a withdrawal of the treatment. This design has good internal validity. Another limitation, common to all baseline designs, is that the observed treatment effect is dependent upon the particular baseline conditions included in the experiment.