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The z test is a statistical test procedure that uses a sample statistic having a normal distribution. Statistical testing is a way of making statistical inferences about unknown population parameters. The z test is usually used to test hypotheses about (a) the mean of a population based on a single sample, (b) the proportion of successes in a population based on a single sample, (c) the difference between the means of two populations based on samples from each population, or (d) the difference between the proportions of successes in two populations based on samples from each population. In the following, capital letters such as X and Z are used to represent random variables and the corresponding lowercase letters such as x and z are used ...

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