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It is an observed interval , in principle different from sample to sample, that frequently includes
\the value of an unobservable parameter of interest if the experiment is repeated.
\How frequently the observed interval contains the parameter is determined by the
\Confidence level or Confidence coefficient.
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confidence level is not used for every study :
For example, a confidence interval can be used to describe how reliable a survey.
\\
A larger sample size normally will lead to a better estimate of the population parameter.
\\
A confidence interval for the proportion in the whole population having the arguments for
the random allocation of treatments to study items are made and of how such intervals might be
\interpreted, are not strictly mathematical.
\Therefore confidence level is not used for every study.
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Reasoning is explained.