Lee et al. (2018)examined selective symptoms (dyspnea, anxiety, depression, and fatigue) to predict their contribution to impaired physical performance in patients with chronic obstructive pulmonary disease (COPD). Dyspnea was measured with FEV1 (forced expiratory volume in 1 second) and anxiety, depression, and fatigue were measured with multi-item Likert scales. Physical performance was measured with the 6-minute walk test (6MWT), which was the distance patients could walk in six minutes. What is the level of measurement for each of the study variables? Rationale?