which therefore can influence results. Nevertheless, both kinds of interviews were conducted to collect exposure data using comparable food items which made them eligible for this analysis.
The case–case study design has been suggested to be a useful tool to analyse surveillance data. It reduces two common methodological limitations which are likely to influence epidemiological studies. Cases and control–cases are identified by the same surveillance system, therefore both are selected in the same manner, which reduces the impact of potential selection bias. In addition, the impact of recall bias caused by the different conditions of memorising by unaffected and infected individuals, is removed (McCarthy and Giesecke, 1999).
A further benefit of such an analysis is the simple and quick way of application. Control–case data is already available by the surveillance system, thus the statistical evaluation can be accomplished as soon as outbreak data is available.