Standard deviation has the same unit as the data set unit.
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Yes. It is the SI unit (that is, the international standard unit) of mass.
Yes, it is part of the metric system, which is used all around the world with the exception of the United States, Liberia, and Myanmar.
So that people's observations can be compared to those of others. For instance, if someone reported that if they gave a horse a bit of barley and it ran very fast, that would hardly be useful if you...