Veterinary Epidemiology and Zoonosis

CLUSTER SAMPLING

• Sometimes, strata are defined by geographical locations such as countries, states, districts, taluks, and villages, veterinary hospitals, veterinary dispensaries or periods of time during which samples are selected. The strata are then termed as 'clusters'.
• It is time consuming and costly method.
• This can be overcome by selecting few clusters, then sampling from few elected clusters is called cluster sampling.
• There are several procedures.

One-stage cluster sampling

• In which selecting all animals in each selected clusters.

Two- stage cluster sampling

• It means selection of sample in more than one stage.
• The samples (all animals) of clusters selected first, followed by sub-sampling of some animals in the clusters.
• In which, clusters are primary units and selected members of the sub-samples are the secondary units.

Multistage cluster sampling

• It is progressively higher levels of sub-sampling.
• First, sampling from the region
• Second, sampling dairy farms in each selected region
• Third, sampling cows on each selected farms
• It is usually a simple random sampling. Sampling can be done even if there is an incomplete list of all members of a population.
• It is a most convenient and inexpensive method. But, information is less precise (for example, contagious disease prevalence tends to be more variable between groups than within them).
• Sometimes impossible if there is no reliable demographic data. In that case, clusters can be defined by map grid.