Kuang et al. BMC Public Health 2012, 12:418 http://www.biomedcentral.com/1471-2458/12/418 Page 4 of 9 Figure 4 The proportion of daily case distribution for eight outbreaks. measles was more than one month, the outbreak signals were injected every two months. Finally, we had eight kinds of outbreak signals, each disease included ten counties, and each county had four outbreak magnitude test datasets, which summed up to 320 test datasets. In theory, 7,200 outbreak signals could be inserted. Exclud- ing the public health emergencies, the actual outbreak signals inserted were 7,088 (Table 1). Incubation classification According to the minimum, maximum, average incuba- tion period of eight diseases [32], eight diseases were clustered into three categories by K-means clustering method: the short incubation period disease (dysentery, Table 1 Summary the number of injected outbreak signals Diseases County Outbreaks should be injected Public health emergencies Actual injected outbreaks Dysentery Scarlet fever Mumps Measles Malaria Typhoid Encephalitis B Hepatitis A Total 10 10 10 10 10 10 10 10 80 960 960 960 480 960 960 960 960 7200 20 1 3 0 0 1 3 0 28 956 948 880 480 960 956 948 960 7008 scarlet fever), the medium incubation period disease (mumps, measles, malaria, typhoid, encephalitis B) and the long incubation period disease (hepatitis A) (Table 2). Performance evaluation Performance comparisons were based on three indica- tors: sensitivity (the proportion of outbreaks the algo- rithm detected), timeliness (the difference between the date of the first true alarm and the beginning date of the outbreak) and false alarm rate (the proportion of non- outbreak days on which the algorithm signal an alarm). For more informative comparisons of the performance in different determinants, we plotted sensitivity-timeli- ness curve [33], which measured the proportion of Table 2 Classification the incubation of diseases with K- means clustering Diseases Incubation period(day) Minimum Maximum Average Dysentery Scarlet fever Mumps Measles Malaria Typhoid Encephalitis B Hepatitis A 1 2 8 6 7 5 10 15 7 12 30 21 30 21 15 45 2 4 18 10 17 9 12 30 Clustering classification Short Short Medium Medium Medium Medium Medium Long