In contrast, the AHM uses an a priori approach to identifying the attributes and specifying their interrelationships in a cognitive model. The RSM using statistical pattern classification where examinees' observed response patterns are matched to pre-determined response patterns that each correspond to a particular cognitive or knowledge state. Each state represents a set of correct and incorrect rules used to answer test items. The focus with the RSM is identification of erroneous rules or misconceptions. The AHM, on the other hand, uses statistical pattern recognition where examinees' observed response patterns are compared to response patterns that are consistent with the attribute hierarchy. The purpose of statistical pattern recognition is to identify the attribute combinations that the examinee is likely to possess. Hence, the AHM does not identify incorrect rules or misconceptions as in the RSM. The AHM uses a construct-centered approach to test development and analysis. Construct-centered emphasizes the central role of the construct in directing test development activities and analysis. The advantage of this approach is that the inferences made about student performance are firmly grounded in the construct specified. Principled test design encompasses 3 broad stages:   Cognitive model development comprises the first stage in the test design process. During this stage, the cognitive knowledge, processes, and skills are identified and organized into an attribute hierarchy or cognitive model. This stage also encompasses validation of the cognitive model prior to the test development stage. Test development comprises the second stage in the test design process. During this stage, items are created to measure each attribute within the cognitive model while also maintaining any dependencies modeled among the attributes. Psychometric analysis comprises the third stage in the test design process. During this stage, the fit of the cognitive model relative to observed examinee responses is evaluated to ascertain the appropriateness of the model to explain test performance. Examinee test item responses are then analyzed and diagnostic skill profiles created highlighting examinee cognitive strengths and weaknesses. An AHM analysis must begin with the specification of a cognitive model of task performance. A cognitive model in educational measurement refers to a "simplified description of human problem solving on standardized educational tasks, which helps to characterize the knowledge and skills students at different levels of learning have acquired and to facilitate the explanation and prediction of students' performance". These cognitive skills, conceptualized as an attribute in the AHM framework, are specified at a small grain size in order to generate specific diagnostic inferences underlying test performance. Attributes include different procedures, skills, and/or processes that an examinee must possess to solve a test item.