Input Values


This page calculates the surveillance sensitivity for complex riskbased surveillance with 2stage sampling, based on actual herdtesting data.
For example, a survey in which high risk herds are preferentially targeted for testing, and
highrisk animals preferentially targeted within selected herds. In addition, different test sensitivities can be applied to highrisk and lowrisk animals in each sampled herd
(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. The analysis is based on
actual sampling results for each herd, assuming that all results are negative.
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).
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) in the highrisk group
being infected, relative to the risk of herds (animals) 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 (or animals) from their respective populations
that are in the highrisk group. Herdspecific values will be used where available, or the population value entered here
will be used where herdspecific values are not available.
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
 Prior confidence of freedom: this is the estimated confidence that the population was free of disease (at the design prevalence)
before the surveillance was done and
is used to estimate confidence of freedom following completion of the surveillance.
 Detailed data on the number of animals tested in each herd, including:
herdrisk group, number of highrisk animals tested, number of lowrisk animals tested,
herd id (optional) and herdspecific population proportion of highrisk animals (optional).
Include only one row of data per herd sampled.
Outputs include:
 The sensitivity of the surveillance system, or in other words, the
probability that the surveillance system would detect at least one infected animal if disease
was present at the specified design prevalence.
 The probability of freedom, or confidence that the
disease is NOT present at the design prevalence (equivalent to the negative predictive value of the surveillance system).
 The sensitivity of the system and confidence of freedom for assumed representative sampling.
 The sensitivity ratio  This indicates how much more sensitivity the riskbased
approach acheives, relative to a representative approach.
 Effective probabilities of infection (EPI) for highrisk and lowrisk herds (and for highrisk and lowrisk animals in each herd). 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.
 Numbers of highrisk and lowrisk herds tested and the total.
 A summary of herdsensitivities achieved in tested herds for riskbased and representative sampling.
 Riskbased and representative herdsensitivities and sensitivity ratio achieved in individual herds.
