Input Values


This page calculates the sample size for simple riskbased surveillance,
for instance, a survey in which a high risk population is targeted.
This analysis assumes that there is no clustering of disease (for instance, we are
working at the herd level), 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).
One risk factor is considered, for which the following information is required:
 The relative risk: this measures the risk of animals being infected in the highrisk
group, relative to the risk of animals being infected in the lowrisk group. For riskbased
surveillance, this should be greater than 1. If analysing biased surveillance (for instance
abattoir testing), the animals tested may have lower probability of being infected
than the rest of the population.
 The population proportion: this is the proportion of animals from the entire population
that are in the highrisk group.
 The surveillance proportion: this is the proportion of animals from the surveillance
that are in the highrisk group.
In addition, the following parameters are required:
 The design prevalence: this is the assumed prevalence of disease, if the disease is
present in the population. It is used as a standard by which the sensitivity of the surveillance
can be evaluated.
 The individual animal test sensitivity: this is the sensitivity of the test performed
on individual animals.
 The target surveillance sensitivity: the
probability that the surveillance system would detect at least one infected animal if disease
was present at the specified design prevalence.
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.
