An ABX test is a method of comparing two kinds of sensory stimuli to identify detectable differences. A subject is presented with two known samples (sample A, the reference, and sample B, an alternative). Of these two samples, one is selected as the unknown sample (sample X), which the subject identifies as being either A or B. If sample X cannot be determined reliably with a low p-value in a predetermined number of trials, then the null hypothesis cannot be rejected and it cannot be proven that there is a perceptible difference between samples A and B.
They can easily be performed as double-blind trials, eliminating any possible unconscious influence from the researcher or supervising technician.
ABX tests are commonly used in evaluations of digital audio data compression methods; sample A is typically a lossless sample without any compression applied, and sample B is a compressed version of A. Audible compression artifacts that indicate a shortcoming in the compression algorithm can be identified with subsequent testing. ABX tests can also be used to compare the different degrees of fidelity loss between two different audio formats at a given bitrate.
ABX test gear utilizing relays to switch between two different hardware paths can help determine if there are perceptual differences in cables and components. Video, audio and digital transmission paths can be compared. If the switching is microprocessor controlled, double-blind tests are possible.
Loudspeaker level and line level audio comparisons could be performed on an ABX test device offered for sale by QSC from 1998 to 2004 as the "ABX Comparator". Other hardware solutions have been fabricated by individuals or organizations for internal testing without being offered for sale.
The foobar2000 and the Amarok audio players support software-based ABX testing, the latter using a third-party script. More ABX software can be found at the PCABX Web Site. Template:Comp-sci-stub de:ABX-Test