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The teststatistic and pvalue
What is the tdistribution and pvalue and how do they tell us how differences between means are likely due to chance or represent real differences. 
Visualize instead of memorize
Animated Flash movies with with audio narration allow you to focus on the concept instead of difficult to understand formulas. 
Sampling Distribution of the Mean Differences
Much of statistical inference comes down to understanding how much a sample mean or the difference between two sample means fluctuate. This distribution is called the sampling distribution and this lesson provides a visual and animated overview of this fundamental concept. 
Concrete examples with memorable visuals
The tutorial takes a concrete example (do street signs slow cars down) and walks through how you can use the 2sample ttest to answer this question. 
Testing your hypothesis using the Null Hypothesis
Statistical tests use a framework called hypothesis testing and the concept of the Null Hypothesis (no difference between means). This lesson covers how to think about what you're comparing in the larger framework of hypothesis testing. 