Actually means the root of the mean of the squares and is an evaluative measurement of a wavefront diagnostic. It describes the smoothness of the surface in terms of deviation from the ideal spherical wavefront and is calculated using the same technique that is used to derive standard deviation. As a general rule of thumb, a Root Mean Square value of .30 microns or greater is an indication that a wavefront-guided ablation is necessary to provide a patient a good refractive surgery outcome.