What is a 'ZTest'
A Ztest is a statistical test used to determine whether two population means are different when the variances are known and the sample size is large. The test statistic is assumed to have a normal distribution and nuisance parameters such as standard deviation should be known in order for an accurate ztest to be performed.
BREAKING DOWN 'ZTest'
A onesample location test, twosample location test, paired difference test and maximum likelihood estimate are examples of tests that can be conducted as ztests. Ztests are closely related to ttests, but ttests are best performed when an experiment has a small sample size. Also, ttests assume that the standard deviation is unknown, while ztests assume that it is known. If the standard deviation of the population is unknown, the assumption that the sample variance equals the population variance is made.

TTest
A statistical examination of two population means. A twosample ... 
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The standard deviation of the sampling distribution of a statistic. ... 
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What assumptions are made when conducting a ttest?
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What is the difference between variance and standard deviation?
Explore the differences between standard deviation and variance. Learn more about how statisticians use these two concepts. Read Answer >> 
What is the difference between standard deviation and variance?
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