The margin of error is a statistic expressing the amount of random
sampling error in the results of a
survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of a census of the entire
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction usi ...
. The margin of error will be positive whenever a population is incompletely sampled and the outcome measure has positive
variance
In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...
, which is to say, the measure ''varies''.
The term ''margin of error'' is often used in non-survey contexts to indicate
observational error
Observational error (or measurement error) is the difference between a measured value of a quantity and its true value.Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP. In statistics, an error is not necessarily a " mista ...
in reporting measured quantities.
Concept
Consider a simple ''yes/no'' poll
as a sample of
respondents drawn from a population
reporting the percentage
of ''yes'' responses. We would like to know how close
is to the true result of a survey of the entire population
, without having to conduct one. If, hypothetically, we were to conduct poll
over subsequent samples of
respondents (newly drawn from
), we would expect those subsequent results
to be normally distributed about
. The ''margin of error'' describes the distance within which a specified percentage of these results is expected to vary from
.
According to the
68-95-99.7 rule, we would expect that 95% of the results
will fall within ''about'' two
standard deviation
In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, whil ...
s (
) either side of the true mean
. This interval is called the
confidence interval
In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...
, and the ''radius'' (half the interval) is called the ''margin of error'', corresponding to a 95% ''confidence level''.
Generally, at a confidence level
, a sample sized
of a population having expected standard deviation
has a margin of error
:
where
denotes the ''quantile'' (also, commonly, a ''
z-score''), and
is the
standard error.
Standard deviation and standard error
We would expect the normally distributed values
to have a standard deviation which somehow varies with
. The smaller
, the wider the margin. This is called the standard error
.
For the single result from our survey, we ''assume'' that
, and that ''all'' subsequent results
together would have a variance
.
:
Note that
corresponds to the variance of a
Bernoulli distribution
In probability theory and statistics, the Bernoulli distribution, named after Swiss mathematician Jacob Bernoulli,James Victor Uspensky: ''Introduction to Mathematical Probability'', McGraw-Hill, New York 1937, page 45 is the discrete probabi ...
.
Maximum margin of error at different confidence levels
For a confidence ''level''
, there is a corresponding confidence ''interval'' about the mean
, that is, the interval