| Standard deviation - Wikipedia, the free encyclopedia |

| | The **standard** deviation is defined as the square root of the variance. |

| | A distinction is made between the **standard** deviation σ (sigma) of a whole population or of a random variable, and the **standard** deviation s of a subset-population sample. |

| | When deciding whether measurements agree with a theoretical prediction, the **standard** deviation of those measurements is of crucial importance: if the mean of the measurements is too far away from the prediction (with the distance measured in **standard** deviations), then we consider the measurements as contradicting the prediction. |

| en.wikipedia.org /wiki/Standard_deviation (1436 words) |