This page calculates the sample size for more complex riskbased surveillance.
This analysis allows for clustering at the herd level, two risk factors (at herd and
animal level) and a factor influencing the performance of the test.
It assumes that these factors are independent and that the effective specificity of the surveillance
system is equal to one (all positives are followed up to ensure that they are not
false positives).
Two risk factor is considered, one at the herd level and one at the animal level. For
each risk factor the following information is required:
 The relative risk: this measures the risk of herds or animals being infected in the highrisk
group, relative to the risk of herds or animals being infected in the lowrisk group. For riskbased
surveillance, this should be greater than 1.
 The population proportion: this is the proportion of herds or animals from the entire population
that are in the highrisk group.
 The surveillance proportion: this is the proportion of herds or hanimals from the surveillance
that are in the highrisk group.
In addition, the following parameters are required:
 The design prevalence at the herd and animal levels: this is the assumed prevalence of disease
(proportion of infected herds and infected animals respectively), if the disease is
present in the population. It is used as a standard by which the sensitivity of the surveillance
can be evaluated.
 For the diagnostic test, performance is depends on a specified factor:
 The proportion of animals in the population in the group with high sensitivity
 The proportion of animals in the surveillance system in the group with high sensitivity
 The sensitivity of the test for animals in the high sensitivity group
 The sensitivity of the test for animals in the low sensitivity group
 The target sensitivity for the surveillance system
The results indicate the required sample size for the surveillance system,
For comparison, the sample size if representative sampling were used is also
shown, along with the savings. This indicates how many fewer animals could be sampled
using the riskbased approach acheives, relative to a representative approach.
