3.2 Non-probability sampling

In non-probability sampling methods, sampling is done without determining a sampling unit’s probability of being sampled; that is, sampling units do not have an equal or known chance of being selected. 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 individuals who present to the medical centre associated with the university where a research study is being conducted. It is common in AMR surveillance programmes and studies, but is highly prone to selection bias. For example, patients attending university medical centres, which often have a large number of different specialties, might have different characteristics from patients attending other types of facilities that offer a small range of specialties.

  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. Purposive sampling is particularly relevant when characterising an emerging type of AMR. For example, shortly after the first reports of colistin resistance in humans, researchers in the Netherlands purposively sampled all travellers attending medical clinics included in a large study, who had recently returned to the Netherlands from an international location. They identified nine out of 1847 returning travellers who had acquired colistin-resistant E. coli infections (Arcilla et al., 2016). This type of study can inform whether surveillance or further research should be initiated, at which point sampling would be done using probability-based methods.

Video 3 summarises what you have learned so far.