A data collection device comprises a first portion and a second portion, a sensor configured to detect rotation of the second portion relative to first portion and a processor arrangement configured to determine a medicament amount expelled by the injection device based on the detected movement. The first portion may be mounted to a component of the injection device that rotates as medicament is expelled. Alternatively, a rotation transfer means, such as a friction wheel, may be used to engage a rotatable component of the injection device and cause the first portion to rotate. The second portion may include outer and inner portions that are axially movable with respect to one another, to provide an electrical contact therebetween and/or to trigger a timer. The timer may monitor a time elapsed since a previous injection in order to alert the user if a subsequent injection is missed or is attempted early.
