Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and here for more.

Quadratic Mean

The quadratic mean is referred to as root mean square (RMS) in electrical engineering and is used to calculate the effective value of and alternating current or voltage:

Due to the averaging of the squares of the observations xi the quadratic mean may not become negative and must not be seen as a "mean" at all - the quadratic mean is in fact a measure of the energy content of a signal.