What is a 'TTest'
A Ttest is a statistical examination of two population means. A twosample ttest examines whether two samples are different and is commonly used when the variances of two normal distributions are unknown and when an experiment uses a small sample size. For example, a ttest could be used to compare the average floor routine score of the U.S. women's Olympic gymnastic team to the average floor routine score of China's women's team.
BREAKING DOWN 'TTest'
The test statistic in the ttest is known as the tstatistic. The ttest looks at the tstatistic, tdistribution and degrees of freedom to determine a p value (probability) that can be used to determine whether the population means differ. The ttest is one of a number of hypothesis tests. To compare three or more variables, statisticians use an analysis of variance (ANOVA). If the sample size is large, they use a ztest. Other hypothesis tests include the chisquare test and ftest.

ZTest
A statistical test used to determine whether two population means ... 
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What percentage of the population do you need in a representative sample?
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