For 10 points, identify this quantity which describes the degree to which a distribution is spread out.
In the analysis of stock prices, the upper and lower Bollinger bands are used to visualize this measure for a security.  Chebyshev's inequality places a 1 over k-square upper bound on the proportion of observations that have a k-tuple value for this quantity. The correlation of variables X and Y is defined as the covariance of X and Y divided by the products of these values for X and Y. T and Z-scores signify multiples of this value for an observation, and this quantity is the square root of the expected value of the square of the distribution minus the mean. For a Gaussian, 2 times the square root of the quantity 2 times natural log of 2, times this quantity, equals the full width at half max. For 10 points, identify this quantity which describes the degree to which a distribution is spread out.
In the analysis of stock prices, the upper and lower Bollinger bands are used to visualize this measure for a security.  
standard deviation