A data collection device (20) for attachment to an injection device (1), such as an injector pen, comprises a sensor arrangement (26) to detect movement of a movable component (70) of the injection device (1) relative to the data collection device (20) during delivery of a medicament by the injection device (1), and a processor arrangement (23) configured to, based on said detected movement, determine a medicament dosage administered by the injection device (1). The processor arrangement (23) may monitor the time that has elapsed since the medicament dosage was administered, and control a display (22) to show the mediacament dosage (22a) and elapsed time to provide a memory aid to the user. In an example embodiment, the sensor arrangement includes an optical encoder and the movable component (70) comprises a plurality of light barrier formations. The movable component (70) may be a number sleeve that provides a visual indication of a dose programmed into the injection device (1).
