Single-case designs apply the logic of time series designs to the evaluation of practice effectiveness with a single case. Baseline the phase of a single-case evaluation design that consists of repeated measures before a new intervention or policy is introduced. The data patterns collected during the baseline phase are compared to the data patterns collected during the intervention phases. To determine whether an intervention is effective, you look for shifts in the trend or pattern of the data that coincide with shifts between the baseline and intervention phases. Single-case designs can increase their control for history by having more than one baseline and intervention phase. Single-system designs or single-case designs are favored by those who seek to remind us that client systems need not be individual subjects. The term single-case designs is more common in social work. The distinguishing feature is that the sample size is one. One of the chief limitations is external validity.