Margin Of Error

Definition: Margin Of Error

Margin Of Error

Noun

1. An index indicating the amount beyond the minimum necessary; "in engineering the margin of safety is the strength of the material minus the anticipated stress".

Specialty Definition: Margin of error

In statistics, including opinion polls and similar surveys, a margin of error is the radius of a confidence interval -- often a 90% confidence interval -- for a proportion of a population.

Example

For example, suppose the quantity of interest is the proportion of voters who will vote "yes" in a referendum. A random sample of the population of voters is taken, and it is found that 60% of voters in the sample will vote "yes". Then the estimated proportion of the whole population who will vote "yes" may be taken to be 60%. If a 3% margin of error is reported, that means a procedure was used that will be within 3% of the proportion to be estimated, 90% of the time. Consequently the interval from 57% to 63% is a 90% confidence interval for the proportion of voters in the whole population who will vote "yes". The radius of that interval is 3%; that is the margin of error.

How to compute a margin of error

Let n be the number of voters in the sample. Suppose them to have been drawn randomly and independently from the whole population of voters. This is perhaps optimistic, but if care is taken it can be at least approximated in reality. Let p be the proportion of voters in the whole population who will vote "yes". Then the number X of voters in the sample who will vote "yes" is a random variable with a binomial distribution with parameters n and p. If n is large enough, then X is approximately normally distributed with expected value np and variance np(1 - p). Therefore

is approximately normally distributed with expected value 0 and variance 1. Consulting tabulated percentage points of the normal distribution reveals that P(-1.645 < Z < 1.645) = 0.9, or, in other words, there is a 90% chance of this event. We have

This is equivalent to

Replacing p in the first and third members of this inequality by the estimated value X/n seldom results in large errors if n is big enough. This operation yields

The first and third members of this inequality depend on the observable X/n and not on the unobservable p, and are the endpoints of the confidence interval. In other words, the margin of error is

Commercial Usage: Margin Of Error

Domain	Title
Books	Margin of Error (Britt Montero Mysteries) [ABRIDGED] (reference) (more book examples)
Source: compiled by the editor from various references; see credits.

Frequency of Internet Keywords: Margin Of Error

The following statistics estimate the number of searches per day across the major English-language search engines as identified by various trade publications. Hyperlinks lead to commercial use of the expression at Amazon.com.

Modern Translations: Margin Of Error

Language		Translations for "margin of error"; alternative meanings/domain in parentheses.
Hungarian		tolerancia (margin, tolerance), hibahatár. (various references)

Pig Latin		arginmay ofay erroray

Spanish		margen de error. (various references)

Swedish		felmargin. (various references)
Source: compiled by the editor from various translation references.

Anagrams: Margin Of Error

Scrabble® Enable2K-Verified Anagrams
	Words within the letters "a-e-f-g-i-m-n-o-o-r-r-r-r"
	-3 letters: forearming.
	-4 letters: reforming, reframing, reroofing.
	-5 letters: aeriform, armigero, armoring, fireroom, informer, orangier, rearming, reinform, reniform, renogram.
Source: compiled by the editor from various references; see credits. SCRABBLE® is a registered trademark. All intellectual property rights in and to the game are owned in the U.S.A and Canada by Hasbro Inc., and throughout the rest of the world by J.W. Spear & Sons Limited of Maidenhead, Berkshire, England, a subsidiary of Mattel Inc. Mattel and Spear are not affiliated with Hasbro.