We see the process of educational measurement as one of defining dimensions of educational progression and locating learners on those dimensions. Such dimensions are variously referred to as proficiency scales or learning metrics. When we measure something using a scale we can express how much of a given attribute is possessed by that something. We use a scale of centimetres or inches to describe the length of an object. We use a scale of hours and minutes to describe an amount of time. This is also the case for educational measurement: when we measure proficiency in a subject area we use a proficiency scale (or learning metric) to describe the extent to which the learner possesses the skills, knowledge and understanding that comprise the area. A learner who is further along the learning metric has greater proficiency in a subject area than a student at a lower point of the learning metric.