3.2 Non-probability sampling

In non-probability sampling methods, sampling is done without determining a sampling unit’s probability of being sampled. Non-probability sampling methods should be avoided, as they introduce substantial bias, and greatly limit the applicability of the findings to the target population. There are two broad types of non-probability sampling methods:

• Convenience sampling is the collection of easily accessible sampling units, such as animals that present to a veterinary clinic, or farms located close to a laboratory with capacity for antimicrobial susceptibility testing (AST).

  Convenience sampling is common in AMR surveillance programmes, but is highly prone to selection bias. For example, farms located close to veterinary laboratories might have different biosecurity practices and other characteristics than farms in more remote areas. Convenience samples are typically poorly representative of the source population, and the findings from convenience samples cannot be generalised to the target population. Therefore, it is difficult to justify convenience sampling, even though it is relatively commonly used. Efforts to identify and select from sampling frames should be promoted over convenience sampling.

• In purposive sampling, units are deliberately selected because they have particular characteristics. Purposive sampling might be appropriate when dealing with a very rare disease or other health-related characteristic, as it can be impractical to use probability-based sampling in these circumstances. Instead, efforts are made to sample as many sampling units that have the disease or characteristic as possible. For example, a study of the prevalence of AMR in E. coli isolates from meat in South Africa made use of both systematic random sampling and purposive sampling (Jaja et al., 2020):

  • A total of 83 and 35 carcasses were sampled in the formal meat sector (FMS) and informal meat sector (INMS) respectively by swabbing the rump, neck, brisket, and flank areas[.] … Systematic random sampling was adopted for the FMS, whereas a purposive sampling technique was adopted for the INMS. The difference in the sampling method is due to the disparity in the number of animals slaughtered in the FMS and INMS.

• Simply put, when there was a small number of animals, all of them were sampled; however, because numbers are much larger in the formal meat sector, it was possible to use systematic random sampling.

Video 3 summarises what you have learned so far.