1-Stage Freedom analysis
# Confidence of population freedom (NPV) for a surveillance system

### Inputs are:

This utility can be used to calculate the confidence of freedom (equivalent to negative predictive value) of a surveillance system. Assumptions are that:

- Specificity of the testing regimen is assumed to be 100% - any positives detected on screening are followed up to confirm or refute the result; and
- All test results (after follow-up) are negative.

- Population-level sensitivity of the surveillance system (SSe for a population, SeH for an individual cluster (herd or flock));
- Prior confidence of population freedom; and
- The probability of introduction (the probability that prevalence will exceed the design prevalence during the surveillance period).

Output of the calculation is the posterior confidence that the population is free, given the negative surveillance data.