Whether a variable is in fact constant—so that it takes on exactly the same value in different observations—is of common concern in statistical work. The question may arise for all the observations in a dataset or for different subsets of the dataset. It may arise because constancy is desirable or because constancy is undesirable, but either way we often need to check simply and quickly. In this column, I discuss several methods for checking. I do not claim to offer a complete analysis of why you might or should care but will give examples arising from experience.

中文翻译：

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讲故事的国家：变量是常数吗？

变量是否实际上是常数（以使其在不同的观察中具有完全相同的值）是统计工作中普遍关注的问题。对于数据集中的所有观测值或数据集的不同子集，可能会出现问题。之所以会出现这种情况，是因为需要恒定性，或者因为不需要恒定性，但是无论哪种方式，我们通常都需要简单快速地进行检查。在本专栏中，我讨论了几种检查方法。我没有要求对为什么或应该关心的问题提供完整的分析，但会举例说明经验。