If the sample ends up with one or more sub-groups that are either over-represented or under-represented, then we say the sample is biased.
The practice of polling only those who you believe will support your cause is sometimes referred to as cherry picking.
Many surveys may have a non-response bias. In this case, a survey that is simply handed out gains few responses when compared to the number of surveys given out. People who are either too busy or simply not interested will be excluded from the results. Non-response bias may be reduced by conducting face-to-face interviews.
Random sampling is a method in which people are chosen out of the blue. In a true random sample, everyone in the population must have the same chance of being chosen. It is important that each person in the population has a chance of being picked.
The number of individuals, or objects, that are surveyed or tested in the study.
Self-selected respondents who tend to have stronger opinions on subjects than others and are more motivated to respond may also cause bias. For this reason phone-in and online polls also tend to be poor representations of the overall population.
A method actively seeking to poll people from many different backgrounds. The population is first divided into different categories (or strata) and the number of members in each category is determined.
A census is an official enumeration of the entire population, with details as to age, sex, occupation, etc.
A destructive study requires that the sample be ruined for its intended use by the study itself. One example is in the tests to see if cars are safe.
Enumerate means to catalogue or list members independently.
Learn how to use surveying and sampling to collect accurate data.