Evidence-appraisal glossary
Standard deviation
Standard deviation measures how far individual values in a dataset typically fall from their mean, expressed in the same units as the data.
Also called: SD.
Researchers report standard deviation alongside a mean to show how tightly or loosely the individual observations cluster around it, so a mean of 10 with a standard deviation of 1 describes a very different group than a mean of 10 with a standard deviation of 8. A common misreading is to confuse it with the standard error: standard deviation describes the spread of the raw data and does not shrink as the sample grows, while standard error describes how precisely a summary statistic is estimated.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.