A large number of subjects taking part in an experiment allow for dividing them into different groups such as the control group (without treatment), the placebo group and the experimental group (with the treatment of interest). In such a design that uses many groups and many subjects in every group, we may use only post-tests. We may even strengthen this kind of experiment using a single-blind (when subjects do not know what drug is receiving) or a double-blind paradigm (when neither subjects nor experimenters know what drug is tested). Another variation of this approach is a crossover study where each subject is tested both for placebo and specific treatment, however in converted design (first drug and second placebo, or contrary first placebo and second drug). When we do not have many subjects willing to participate in an experiment, we may design a single-case study that may be conducted on a few participants. In such a situation the experimental design would need both the pre- and post-test to demonstrate the effect of the treatment on a given study case.