1.1 Why do we need to sample?

Have you ever taken part in a census, such as a national human population census, or a livestock census? A census involves collecting data from every single unit (such as a human, animal, farm) in the population. Conducting a true census is a very resource-intensive activity. What happens if we would like to answer a question such as:

Is it necessary to collect data on every single chicken in a country to answer this question? In most cases, it is impractical to conduct a census, especially when we need to collect specimens, such as blood, urine or faecal samples, from every single person or animal in the population.

Fortunately, for the majority of research or surveillance, it is not necessary to conduct a census. Instead, we can select an appropriate sample of subjects from the population. Sampling allows us to make inferences about a larger population. But we can’t just pick any animals we happen to find and expect that this ‘sample’ allows us to make inferences (apply the findings) to the entire population; instead, there are several steps we have to go through to ensure that our sample is representative (accurately reflects the characteristics) of the broader population that we are interested in. Going through these steps is the focus of this section.