  Tracking physical activity reliably is becoming central to many research
efforts. In the last years specialized hardware has been proposed to measure
movement. However, asking study participants to carry additional devices has
drawbacks. We focus on using mobile devices as motion sensors. In the paper we
detail several issues that we found while using this technique in a
longitudinal study involving hundreds of participants for several months. We
hope to sparkle a lively discussion at the workshop and attract interest in
this method from other researchers.
