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Risk-based surveillance

Complex 2-stage risk-based surveillance - calculation of surveillance sample size

Herd-level risk factor

Animal-level risk factor

Diagnostic test

This page calculates the sample size for more complex 2-stage risk-based 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 high-risk and low-risk animals (i.e. sensitivity is based on risk-group). 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 high-risk group, relative to the risk of herds or animals being infected in the low-risk group. For risk-based 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 high-risk group; and
  • The surveillance proportion: this is the proportion of herds or animals from the surveillance that are in the high-risk 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; and
    •   . The sensitivity of the test for animals in the low risk group.
  • The target herd-sensitivity for herds tested in the surveillance system; and
  • The overall target sensitivity for the surveillance system.

Outputs include:

  • The required sample size for high and low-risk herds and overall;
  • The required sample size within sampled herds for high and low-risk 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 risk-based sampling. This indicates how many fewer herds and animals could be sampled using the risk-based approach, relative to a representative approach; and
  • Effective probabilities of infection (EPI) for high-risk 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 high-risk 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.