Why might “person on the street” interviews, while interesting, not tell much about how the population of one city views police brutality? How can sampling assist?

Question

What will be an ideal response?

Answer 1

Probability sampling methods are those in which the probability of selection is known and is not zero (so there is some chance of selecting each element). These methods randomly select elements and therefore have no systematic bias; nothing but chance determines which elements are included in the sample. This feature of probability samples makes them much more desirable than nonprobability samples when the goal is to generalize to a larger population. Even though a random sample has no systematic bias, it certainly will have some sampling error due to chance.
Because they do not disproportionately select particular groups within the population, random samples that are successfully implemented avoid systematic bias. The Gallup Poll is a good example of the accuracy of random samples. For example, in 2012, the final Gallup prediction of 54% for Obama was within three percentage points of his winning total of 51%. The four most common methods for drawing random samples are simple random sampling, systematic random sampling, stratified random sampling, and multistage cluster sampling.