Figure 5.

Impact on detection capabilities of different stages in syndromic surveillance.

Surveillance stage	Potential problems causing…
	Failure to detect	False alarms	Delays
data collection, storage and extraction	Sentinel system does not cover location of ‘event’	Data quality, duplicates, test data etc.	Delay between exposure and presenting to health care
aggregation to syndromic indicators	Symptoms not covered by existing indicators	Similar symptoms caused by other reasons	Data processing
application of detection algorithms	Alarm thresholds set too high (no alarm) or too low (more alarms than can be analysed)	Alarm thresholds set too low	Computational complexity also alarm volume impacts on next stage
interpretation, reporting and taking action	Failure to take appropriate public health action following alarm	Failure to distinguish between false alarm and potential health threat	Staff time, waiting for ‘repeat’ alarms to provide confirmation, decision-making processes