A lack of independence between samples; for example, because they are taken from the same farm. Infection in a country, zone or compartment usually clusters rather than being uniformly or randomly distributed through a population. Clustering may occur at a number of different levels, such as a cluster of infected animals within a herd or flock, a cluster of pens in a building, or a cluster of farms in a compartment. It should be taken into account in the design of surveillance activities and the statistical analysis of surveillance data, at least at what is judged to be the most significant level of clustering for the particular animal population and infection.