Saskia Bakker Panos Markopoulos Yvonne de Kort OPOS: an observation scheme for evaluating head-up play. 33-42 2008 NordiCHI http://doi.acm.org/10.1145/1463160.1463165 conf/nordichi/2008 db/conf/nordichi/nordichi2008.html#BakkerMK08