In practice, when conducting surveys to substantiate freedom from disease in large populations twostage sampling strategies are often used in order to account for herdlevel clustering of diseases. With twostage sampling the questions that need to be addressed are a) how many herds need to be sampled and b) how many animals per herd need to be tested. Using a modified hypergeometric formula the optimal sample sizes can elegantly be computed, while incorporating imperfect diagnostic tests and finite populations; see Cameron and Baldock (1998a,b).
In the package FFD, tools for calculating optimal sample sizes (on animal and herd level) using sampling strategies "individual sampling" or "limited sampling" (see Ziller, Selhorst, Teuffert, Kramer, and Schlüter (2002)) are implemented.
Both these sampling strategies have their merits, which can be analyzed and compared using the package FFD. Cost optimal sampling strategies can be determined finding the optimal herd sensitivity or sample limit, respectively. The package provides an easytouse graphical user interface (GUI). More advanced users can, however, make use of the provided S4classes and methods, as well as the underlying conventional functions in order to fully make use of the maximum flexibility offered.
The latest release of the package FFD now offers the possibility to use riskbased sampling. Targeting highrisk population strata, the overall sample size and the cost of the surveys can be reduced, while still maintaining the same overall significance level.
Features

To install this package directly within R type:
install.packages("FFD", repos="http://RForge.Rproject.org")
or download source code or binary from RForge.