Tracking Ability: Defining Trackers for Measuring Educational Progress

In measurements that extend over longer periods of time, we might expect all sorts of changes in model parameters, such as ability and item difficulty. We define trackers as instruments with specific properties to deal with such changing parameters. First, trackers should allow for estimating dynamically changing parameters, adapting to possible changes in ability or item difficulty. Second, if no change occurs for some time, trackers should provide unbiased estimates with a known error variance. Such trackers retain the strengths of both state space models and rating systems, while resolve some of their weaknesses. These properties are especially suitable for educational measurement applications such as tracking individual progress or any aggregate thereof, as in reporting survey research.