The Likelihood-ratio test is another IRT based method for assessing DIF. This procedure involves comparing the ratio of two models. Under model (Mc) item parameters are constrained to be equal or invariant between the reference and focal groups. Under model (Mv) item parameters are free to vary. The likelihood function under Mc is denoted (Lc) while the likelihood function under Mv is designated (Lv). The items constrained to be equal serve as anchor items for this procedure while items suspected of DIF are allowed to freely vary. By using anchor items and allowing remaining item parameters to vary, multiple items can be simultaneously assessed for DIF. However, if the likelihood ratio indicates potential DIF, an item-by-item analysis would be appropriate to determine which items, if not all, contain DIF. The likelihood ratio of the two models is computed by