Veterinary Epidemiology and Zoonosis

TERMS USED IN SAMPLING TECHNIQUES

Target population

• It is the total population (i.e., population at risk) about which information is required.

Study population

• It is the population from which a sample is drawn.
• Example: If an investigation of parvoviral enteritis in Pommeranian is undertaken, then ideally all Pommeranian animals (the target population) should be sampled, although it may only be possible to investigate in that particular breed at dog shows or attending veterinary hospitals (the study population).

Elementary units

• Each unit in the study population is the elementary unit, which cannot be divided further. 

Stratum

• A collection of elementary units grouped according to a common characteristic feature. Example: A dairy farm is a stratum comprising cows.

Sampling frame

• Before a sample is taken, members of the study population must be identified by constructing a list (i.e. sampling frame).

Sampling unit

• Each member of the sampling frame is a sampling unit.

Sampling fraction

• It is the ratio of sample size to study population size.
• If 10 animals were chosen from 1000 animal population, the sampling fraction would be 1%.
• Veterinary sampling frames include lists of abattoirs, farms, veterinary hospitals, etc.