This paper discusses similarity tests using forced-choice methods in terms of Thurstonian discriminal distance, d′, a measure of pure sensory difference or similarity. Critical values are derived for the similarity tests using the 2-alternative forced choice (AFC), the duo-trio, the 3-AFC and the triangle methods in terms of d′. Power analysis is conducted for the similarity tests using the four forced-choice methods. Three approaches are used to estimate sample sizes for the tests. The approaches are based, respectively, on exact binomial distribution, normal approximation with a continuity correction and Monte Carlo simulation. S-Plus and R codes were developed and provided for calculation and estimation of the critical values, testing powers and sample sizes for the tests.