This paper presents a conceptualization of test bias for standardized ability tests which is based on multidimensional, non-parametric, item response theory. An explanation of how individually-biased items can combine through a test score to produce test bias is provided. It is contended that bias, although expressed at the item level, should be studied at the test level. The model postulates an intended-to-be-measured target ability and nuisance determinants whose differing ability distributions across examinee group cause bias. Multiple nuisance determinants can produce item bias cancellations, resulting in little or no test bias. Detection of test bias requires a valid subtest, whose items measure only target ability. A long-test viewpoint of bias is also developed. Four graphs and a 30-item list of references are included. (Author/SLD)