Input Values


This page calculates the sample size for simple riskbased surveillance with 2stage sampling,
for instance, a survey in which a high risk herds are preferentially targeted for testing, but with
representative sampling of animals within selected herds.
The analysis assumes 2stage sampling to account for clustering of disease (for example
at the herd, flock or village 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 herds in the highrisk group
being infected, relative to the risk of herds in the lowrisk group being infected. For riskbased
surveillance, this should usually be greater than 1.
 The population proportion: this is the proportion of herds from the entire population
that are in the highrisk group.
 The surveillance proportion: this is the proportion of herds 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. Values must be entered for both herd and animallevel design prevalence.
 The individual animal test sensitivity: this is the sensitivity of the test performed
on individual animals.
 The target herdsensitivity: the desired confidence for each herd sampled that the disease would be detected, if
the herd was infected at the animallevel design prevalence. For this analysis herd sensitivity
is assumed to be constant across all herds and herds are assumed to be large relative to sample size.
 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.
Outputs include:
 The required sample size for high and lowrisk herds and overall
 The required sample size within sampled herds and overall.
 For comparison, the sample sizes if representative sampling were used.
 The percentage reduction (savings) in the number of herds required for riskbased
sampling. This indicates how many fewer herds could be sampled
using the riskbased approach, relative to a representative approach.
 The effective probability of infection (EPI) for highrisk herds. EPI values approaching 100% suggest
that, based on the values used for relative risk, population proportions and design prevalence, close to 100% of herds (or animals) in the highrisk group are
expected to be infected. If this is unreasonable you may need to review the input values.
Values over 100% mean that the model is invalid and processing will be stopped, with an error message.
Input values must be changed to ensure EPI values are appropriate.
