Risk-based surveillance

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

Relative risk

Population proportion in high risk group

Surveillance proportion in high risk group

Herd-level design prevalence

Animal-level design prevalence

Test sensitivity

Target herd-sensitivity

Target surveillance sensitivity

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This page calculates the sample size for simple risk-based surveillance with 2-stage sampling, for instance, a survey in which high risk herds are preferentially targeted for testing, but with representative sampling of animals within selected herds.

The analysis assumes 2-stage 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 high-risk group being infected, relative to the risk of herds in the low-risk group being infected. For risk-based 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 high-risk group; and
• The surveillance proportion: this is the proportion of herds from the surveillance that are in the high-risk 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 animal-level design prevalence;
• The individual animal test sensitivity: this is the sensitivity of the test performed on individual animals;
• The target herd-sensitivity: the desired confidence for each herd sampled that the disease would be detected, if the herd was infected at the animal-level 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; and
• 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 low-risk 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 risk-based sampling. This indicates how many fewer herds could be sampled using the risk-based approach, relative to a representative approach; and
• The effective probability of infection (EPI) for high-risk 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 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.