I have problem understanding following example in parentheses. Maybe you can help me.
Ideally, researchers know the full extent of the population they want to study, and they can select a sample from this population at random. Statisticians can calculate the probability that such random samples represent the population; this is usually expressed in terms of sampling error (for example, there might be a 95 percent probability that the distribution of responses in a sample will be within 3 percent of the distribution in the population).