A standard deviation of zero means that all the data points are the same value.
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Standard deviation can only be zero if all the data points in your set are equal. If all data points are equal, there is no deviation. For example, if all the participants in a survey coincidentally were all 30 years old, then the value of age would be 30 with no deviation. Thus, there would also be no standard deviation.A data set of one point (small sample) will always have a standard deviation of zero, because the one value doesn't deviate from itself at all.!
The mean and variance are equal in the Poisson distribution. The mean and std deviation would be equal only for the case of mean = 1. See related link.
The mean would be negative, but standard deviation is always positive.
Standard deviation is a number and you would divide it in exactly the same way as you would divide any other number!
The standard deviation, in itself, cannot be high nor low. If the same measurements were recorded using a unit that was a ten times as large (centimetres instead of millimetres), the standard deviation for exactly the same data set would be 1.8. And if they were recorded in metres the sd would be 0.018