The margin of error is the statistical number that defines the chances that a sample differs from its population. It is used as a theoretical concept in mathematics, and is widely used in any form of research, usually political or sociological, that requires extrapolating information from a limited amount of subjects onto a larger population.

Since this is election season, and the most common place that people will encounter the phrase "margin of error" is in the context of political polling, I will talk about that application of the concept, rather than the mathematical formalism, which I don't have the knowledge to speak on anyway. In most political polls in the United States with a realistic sample size (between 500 and 1500 people, usually) the margin of error is between 3 and 5%. Since most American elections, both on the whole and in the states that are most important in the electoral college, are decided by a smaller percentage than that, most polls are not conclusive sources of information. They are usually only useful in the context of other polls, or of the previous electoral history of a state.

The other thing to remember about a reported margin of error is that a margin of error is a mathematical formalism, that defines the chance of error in a truly randomly selected sample. The margin of error is what is left over when a poll has otherwise perfected its methodology. In practice, in political polls, even the most conscientious, least biased polling organizations have less-than-perfect methodologies. Everything from when to call, to how many questions to ask, to whether a call uses a live person or a recording can skew the answers somewhat. Therefore, the mathematical margin of error may be secondary to the non-technical "margin of error" introduced by methodology.

So, a "margin of error", in political polling, is a technical way to describe the known unknowns. However, it is always best to remember that along with that, there are also many unknown unknowns.

Log in or register to write something here or to contact authors.