This tool is used to test the null hypothesis that there is no difference between two population means against either one-sided or two-sided alternative hypotheses.

此工具用于检验两个总体平均值之间不存在差异的空值假设，而不是单方或双方的其他假设。

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"There's no known source of caffeine in temperate zones, so there's nothing we can suggest as a sort of alternative hypothesis at this point except for a human source, " she said.

CNN: Researchers find caffeine in waters off Oregon