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Complex risk-based surveillance - calculation of surveillance sample size
This page calculates the sample size for more complex risk-based surveillance.
This analysis allows for clustering at the herd level, two risk factors (at herd and animal level) and a factor influencing the performance of the test. It assumes that these 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 should be 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 a specified factor:
- . The proportion of animals in the population in the group with high sensitivity;
- . The proportion of animals in the surveillance system in the group with high sensitivity;
- . The sensitivity of the test for animals in the high sensitivity group; and
- . The sensitivity of the test for animals in the low sensitivity group.
- The target sensitivity for the surveillance system.
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 risk-based approach, relative to a representative approach.