  Fluctuations in nucleon positions can affect the spatial eccentricity of the
overlap zone in nucleus-nucleus collisions. We show that elliptic flow should
be scaled by different eccentricities depending on which method is used for the
flow analysis. These eccentricities are estimated semi-analytically. When $v_2$
is analyzed from 4-particle cumulants, or using the event plane from directed
flow in a zero-degree calorimeter, the result is shown to be insensitive to
eccentricity fluctuations.
