Input Values


This page calculates the sample size for more complex 2stage riskbased surveillance.
This analysis allows for clustering at the herd level, two risk factors (at herd and
animal level) and/or a factor influencing the performance of the test. In addition, different test sensitivities can be applied to highrisk and lowrisk animals
(i.e. sensitivity is based on riskgroup). If test sensitivity varies between groups but there is no difference in
risk of disease use a relative risk value of 1.
It assumes that the risk 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 factors are 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 is usually 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 animals 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 depends on risk group:
 The sensitivity of the test for animals in the high risk group
 The sensitivity of the test for animals in the low risk group
 The target herdsensitivity for herds tested in the surveillance system
 The overall target sensitivity for the surveillance system
Outputs include:
 The required sample size for high and lowrisk herds and overall
 The required sample size within sampled herds for high and lowrisk animals and overall.
 For comparison, the sample sizes if representative sampling were used.
 The percentage reduction (savings) in the number of herds and animals required for riskbased
sampling. This indicates how many fewer herds and animals could be sampled
using the riskbased approach, relative to a representative approach.
 Effective probabilities of infection (EPI) for highrisk groups at both herd and animal levels are also shown. 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.
