A census involves collecting data for the entire population of interest. This is problematic for several reasons, including:
- It can be difficult to complete a census: there always seem to be some individuals who are hard to locate or hard to measure. And these difficult-to-find people may have certain characteristics that distinguish them from the rest of the population.
- Populations rarely stand still. Even if you could take a census, the population changes constantly, so it’s never possible to get a perfect measure.
- Taking a census may be more complex than sampling.
Sampling involves measuring a subset of the population of interest, usually randomly.