IRT & CAT

why we use IRT & CAT in this moment? It is because we didnt have clear definition of functional outcome or delineation between intrusments that assess functional outcomes. In the field of rehabilitation have many,different types of intrusments are overlapping with same function. So ,the contemporary measurement techniques have some functions better than traditional measurement such as it can unity,completeness and consistency issues.Following the last discription, it also can saves time in busy clinical settings. Question :how to achieve the functions that mention in the above sentences. Or how to prove that the CAT &IRThave such function?