In Frank Attix's book on Radialogical Physics Equation 1.2a reads σ = √(E) ≈ √(μ) σ= standard deviation of a single random measurement Where E is the expecation value of a stochastic process which approaches μ (μ is the average of measured values) as the number of measured values becomes vary large (∞) I agree with the how the mean and expectation value approach each other, I do not see how the standard deviation is the square root of the expectation value. Isn't the stdv σ = √[E(x^2)-E(x)^2] = √[E(x-μ)^2] He continues with the following example: A detector makes 10 measurements, for each measurement the average number of rays detected (counts) per measurement is 10^5. He writes that the standard deviation of the mean is √[E(x)/n]≈√[μ/n] = √[(10^5)/10] where n is the number of measurements. I agree that the standard deviation of the mean is related to the standard deviation by σ'=σ/√(n), but once again I'm not sure how he gets the standard deviation itself. Help!