A data collection device for attachment to an injection device, such as an injector pen, includes a sensor arrangement to detect movement of a movable component of the injection device relative to the data collection device during delivery of a medicament by the injection device, and a processor arrangement configured to, based on said detected movement, determine a medicament dosage administered by the injection device. The processor arrangement may monitor the time that has elapsed since the medicament dosage was administered, and control a display to show the mediacament dosage 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 includes a plurality of light barrier formations. The movable component may be a number sleeve that provides a visual indication of a dose programmed into the injection device.