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In statistics, an estimator is a function of the known sample data that is used to estimate an unknown population parameter; an estimate is the result from the actual application of the function to a particular set of data. Many different estimators are possible for any given parameter. Some criterion is used to choose between the estimators, although it is often the case that a criterion cannot be used to clearly pick one estimator over another. To estimate a parameter of interest (e.g., a population mean, a binomial proportion, a difference between two population means, or a ratio of two population standard deviation), the usual procedure is as follows:
1- Select a random sample from the population of interest.
2- Calculate the point estimate of the parameter.
3- Calculate a measure of its variability, often a confidence interval.
4- Associate with this estimate a measure of variability.
For a point estimator of parameter ,
- The error of is
- The bias of is defined as
- is an unbiased estimator of θ if and only if for all θ, or, equivalently, if and only if for all θ.
- The mean squared error of is defined as
- i.e. mean squared error = variance + square of bias.
A consistent estimator is an estimator that converges in probability to the quantity being estimated as the sample size grows.
It is called strongly consistent, if it converges almost surely to the true value.
The quality of an estimator is generally judged by its mean squared error.
However, occasionally one chooses the unbiased estimator with the lowest variance. Efficient estimators are those that have the lowest possible variance among all unbiased estimators. In some cases, a biased estimator may have a uniformly smaller mean squared error than does any unbiased estimator, so one should not make too much of this concept. For that and other reasons, it is sometimes preferable not to limit oneself to unbiased estimators; see bias (statistics). Concerning such "best unbiased estimators", see also Cramér-Rao inequality, Gauss-Markov theorem, Lehmann-Scheffé theorem, Rao-Blackwell theorem.
See: Robust estimator, Robust statistics
Often, estimator are due to restrictions (restricted estimators).
- Maximum likelihood
- Method of moments, generalized method of moments
- Cramér-Rao inequality
- Minimum mean squared error (MMSE)
- Maximum a posteriori (MAP)
- Minimum variance unbiased estimator (MVUE)
- Best linear unbiased estimator (BLUE)
- Unbiased estimators — see bias (statistics).
- Particle filter
- Markov chain Monte Carlo (MCMC)
- Kalman filter
- Wiener filter
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