We limited our investigation to the food items eaten
in the school canteen on 20 and 21 March. From the
information received from the school, we knew that the
outbreak came from a point source, had an extremely
rapid onset and symptoms were short-lived. This
directed our suspicions towards a bacterial toxin or aviral infection, all with short incubation periods. We
felt that investigating food items eaten in the canteen
over two days back in time would have rendered the
questionnaire unnecessarily long, hindering response
rates. We also considered that those responses would
have been subject to considerable recall bias.