(Student's) t distribution
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As the degree of freedom of the t distribution increases, it approaches the standard normal distribution.
As the degree of freedom of t distribution decreases, it approaches the standard normal distribution.
When the population variance is unknown, standardize the sample mean with unbiased variance estimator. Then, the standardized statistic follows the t distribution.
When a random variable T follows the t distribution, the probability Pr(ITI>1.96) is approximately 0.05 irrespective of the degree of freedom.